runstate: ignore exit request in finish migrate state

Trying to reboot a VM while a migration is running can
move to the prelaunch state (because of the reset) while
the runstate is in finish migrate state.
As the logical step after the finish migrate is postmigrate,
this can create an invalid state transition from prelaunch state
to postmigrate state and this raises an error and aborts:

    invalid runstate transition: 'prelaunch' -> 'postmigrate'

As we are not able to manage reset in finish migrate state the
best we can do is to ignore any changes and delay them until
the next state which should be postmigrate and which should allow
this kind of transition.

Reported-by: Lukáš Doktor <ldoktor@redhat.com>
Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20191017101806.3644-1-lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>