multipassd segfaults on launch and delete --purge on same instance

[townsend2010]

@gerboland posted the following traceback in #752:

#0  QProcess::state (this=0x0) at io/qprocess.cpp:1756
#1  0x0000555555c3cddd in multipass::QemuVirtualMachine::ensure_vm_is_running() ()
#2  0x0000555555c41d25 in void std::__invoke_impl<void, void (multipass::QemuVirtualMachine::*&)(), multipass::QemuVirtualMachine*&>(std::__invoke_memfun_deref, void (multipass::QemuVirtualMachine::*&)(), multipass::QemuVirtualMachine*&) ()
#3  0x0000555555c41972 in std::__invoke_result<void (multipass::QemuVirtualMachine::*&)(), multipass::QemuVirtualMachine*&>::type std::__invoke<void (multipass::QemuVirtualMachine::*&)(), multipass::QemuVirtualMachine*&>(void (multipass::QemuVirtualMachine::*&)(), multi
pass::QemuVirtualMachine*&) ()
#4  0x0000555555c4140f in void std::_Bind<void (multipass::QemuVirtualMachine::*(multipass::QemuVirtualMachine*))()>::__call<void, , 0ul>(std::tuple<>&&, std::_Index_tuple<0ul>) ()
#5  0x0000555555c40e09 in void std::_Bind<void (multipass::QemuVirtualMachine::*(multipass::QemuVirtualMachine*))()>::operator()<, void>() ()
#6  0x0000555555c40777 in std::_Function_handler<void (), std::_Bind<void (multipass::QemuVirtualMachine::*(multipass::QemuVirtualMachine*))()> >::_M_invoke(std::_Any_data const&) ()
#7  0x0000555555c5fa44 in std::function<void ()>::operator()() const ()
#8  0x0000555555c5cf12 in multipass::utils::wait_until_ssh_up(multipass::VirtualMachine*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::function<void ()> const&)::{lambda()#1}::operator()() const ()
#9  0x0000555555c5de57 in void multipass::utils::try_action_for<multipass::utils::wait_until_ssh_up(multipass::VirtualMachine*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::function<void ()> const&)::{lambda()#2}&, multipass::utils::wait_until_ssh_up(multip
ass::VirtualMachine*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::function<void ()> const&)::{lambda()#1}&>(multipass::utils::wait_until_ssh_up(multipass::VirtualMachine*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::function<void ()> const&):
:{lambda()#2}&, std::chrono::duration<long, std::ratio<1l, 1000l> >, multipass::utils::wait_until_ssh_up(multipass::VirtualMachine*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::function<void ()> const&)::{lambda()#1}&) ()
#10 0x0000555555c5d200 in multipass::utils::wait_until_ssh_up(multipass::VirtualMachine*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::function<void ()> const&) ()
#11 0x0000555555c3d24c in multipass::QemuVirtualMachine::wait_until_ssh_up(std::chrono::duration<long, std::ratio<1l, 1000l> >) ()
#12 0x0000555555bad000 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > multipass::Daemon::async_wait_for_ssh_and_start_mounts_for<multipass::LaunchReply>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > c
onst&, grpc::ServerWriter<multipass::LaunchReply>*) ()
#13 0x0000555555bd3797 in QtConcurrent::StoredMemberFunctionPointerCall2<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, multipass::Daemon, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cx
x11::basic_string<char, std::char_traits<char>, std::allocator<char> >, grpc::ServerWriter<multipass::LaunchReply>*, grpc::ServerWriter<multipass::LaunchReply>*>::runFunctor() ()
#14 0x0000555555bd3611 in QtConcurrent::RunFunctionTask<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::run() ()
#15 0x00007ffff63332b2 in QThreadPoolThread::run (this=0x7fffb80155b0) at thread/qthreadpool.cpp:99
#16 0x00007ffff633617d in QThreadPrivate::start (arg=0x7fffb80155b0) at thread/qthread_unix.cpp:368
#17 0x00007ffff69dc6db in start_thread (arg=0x7fffc9ffb700) at pthread_create.c:463
#18 0x00007ffff567b88f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95