Don't start a nested job in qemuMigrationPrepareAny

Don't start a nested job in qemuMigrationPrepareAny

This nested job is canceled by the first ExitMonitor call (even though
it was not created by the corresponding EnterMonitor call), and
again in qemuMigrationPrepareAny if qemuProcessStart failed.
This can lead to a crash if the vm object was disposed of before calling
qemuDomainRemoveInactive:
0  ..62bc in virClassIsDerivedFrom (klass=0xdeadbeef,
   parent=0x7ffce4cdd270) at util/virobject.c:166
1 ..6666 in virObjectIsClass at util/virobject.c:362
2 ..66b4 in virObjectLock at util/virobject.c:314
3 ..477e in virDomainObjListRemove at conf/domain_conf.c:2359
4 ..7a64 in qemuDomainRemoveInactive at qemu/qemu_domain.c:2087
5 ..956c in qemuMigrationPrepareAny at qemu/qemu_migration.c:2469

This was added by commit e4e2822, exposed by 5a4c237 and c7ac251.

https://bugzilla.redhat.com/show_bug.cgi?id=1018267