Lock the VM on start. If the machine is in the "starting" state we know
that a previous start has failed and guide the user into resolving the
issue.
Concurrent starts will busy wait and return the expected "already
running" error.
NOTE: this change is only looking at the start issue (#18662). Other
commands such as stop and update should also lock and will be updated
in a future change. I expect the underlying issue to apply to all
machine providers, not only QEMU. It's desirable to aim for extending
the machine interface to also allow to `Lock()` and `Unlock()`. After
acquiring the lock, the VM should automatically be reloaded/updated.
[NO NEW TESTS NEEDED]
Fixes: #18662
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>
c341a0ffe0