virQEMUCapsInitQMP: Don't dispose locked @vm

virQEMUCapsInitQMP: Don't dispose locked @vm

When creating qemu capabilities, a dummy virDomainObj is created just
because our monitor code expects that. However, the object is created
locked already. Then, under cleanup label, we simply unref the object
which results in whole domain object to be disposed. The object lock
is destroyed subsequently, but hey - it's still locked:

==24845== Thread #14's call to pthread_mutex_destroy failed
==24845==    with error code 16 (EBUSY: Device or resource busy)
==24845==    at 0x4C3024E: pthread_mutex_destroy (in /usr/lib64/valgrind/vgpreload_helgrind-amd64-linux.so)
==24845==    by 0x531F72E: virMutexDestroy (virthread.c:83)
==24845==    by 0x5302977: virObjectLockableDispose (virobject.c:237)
==24845==    by 0x5302A89: virObjectUnref (virobject.c:265)
==24845==    by 0x1DD37866: virQEMUCapsInitQMP (qemu_capabilities.c:3397)
==24845==    by 0x1DD37CC6: virQEMUCapsNewForBinary (qemu_capabilities.c:3481)
==24845==    by 0x1DD381E2: virQEMUCapsCacheLookup (qemu_capabilities.c:3609)
==24845==    by 0x1DD30F8A: virQEMUCapsInitGuest (qemu_capabilities.c:744)
==24845==    by 0x1DD31889: virQEMUCapsInit (qemu_capabilities.c:1020)
==24845==    by 0x1DD7DD36: virQEMUDriverCreateCapabilities (qemu_conf.c:888)
==24845==    by 0x1DDC57C0: qemuStateInitialize (qemu_driver.c:803)
==24845==    by 0x53DC743: virStateInitialize (libvirt.c:777)
==24845==

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>