qmp: forbid qmp_cont in RUN_STATE_FINISH_MIGRATE

qmp: forbid qmp_cont in RUN_STATE_FINISH_MIGRATE

qmp_cont in RUN_STATE_FINISH_MIGRATE may lead to moving vm to
RUN_STATE_RUNNING, before actual migration finish. So, when migration
thread will try to go to RUN_STATE_POSTMIGRATE, assuming transition
RUN_STATE_FINISH_MIGRATE->RUN_STATE_POSTMIGRATE, it will crash, as
current state is RUN_STATE_RUNNING, and transition
RUN_STATE_RUNNING->RUN_STATE_POSTMIGRATE is forbidden.

Reported-by: Max Reitz <[email protected]>
Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>