qemu: Avoid double free of VM

qemu: Avoid double free of VM

One of my previous patches (c7ac2519b7f) did try to fix the issue when
domain dies too soon during migration. However, this clumsy approach was
missing removal of qemuProcessHandleMonitorDestroy resulting in double
unrefing of mon->vm and hence producing the daemon crash:

==11843== Invalid read of size 4
==11843==    at 0x50C28C5: virObjectUnref (virobject.c:255)
==11843==    by 0x1148F7DB: qemuMonitorDispose (qemu_monitor.c:258)
==11843==    by 0x50C2991: virObjectUnref (virobject.c:262)
==11843==    by 0x50C2D13: virObjectFreeCallback (virobject.c:388)
==11843==    by 0x509C37B: virEventPollCleanupHandles (vireventpoll.c:583)
==11843==    by 0x509C711: virEventPollRunOnce (vireventpoll.c:652)
==11843==    by 0x509A620: virEventRunDefaultImpl (virevent.c:274)
==11843==    by 0x520D21C: virNetServerRun (virnetserver.c:1112)
==11843==    by 0x11F368: main (libvirtd.c:1513)
==11843==  Address 0x13b88864 is 4 bytes inside a block of size 136 free'd
==11843==    at 0x4A07F5C: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==11843==    by 0x5079A2F: virFree (viralloc.c:580)
==11843==    by 0x50C29E3: virObjectUnref (virobject.c:270)
==11843==    by 0x114770E4: qemuProcessHandleMonitorDestroy (qemu_process.c:1103)
==11843==    by 0x1148F7CB: qemuMonitorDispose (qemu_monitor.c:257)
==11843==    by 0x50C2991: virObjectUnref (virobject.c:262)
==11843==    by 0x50C2D13: virObjectFreeCallback (virobject.c:388)
==11843==    by 0x509C37B: virEventPollCleanupHandles (vireventpoll.c:583)
==11843==    by 0x509C711: virEventPollRunOnce (vireventpoll.c:652)
==11843==    by 0x509A620: virEventRunDefaultImpl (virevent.c:274)
==11843==    by 0x520D21C: virNetServerRun (virnetserver.c:1112)
==11843==    by 0x11F368: main (libvirtd.c:1513)

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