qemu_process: Initialize domain definition for QMP query

When constructing QMP capabilities we allocate a dummy domain
object to pass to qemuMonitorOpen(). However, after 75dd5958
the function also expects domain definition to be allocated for
the domain object. The referenced commit already fixed
qemumonitortestutils.c but forgot to fix the other caller:
qemuProcessQMPConnectMonitor().
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_process.c

@@ -8654,7 +8654,8 @@ qemuProcessQMPConnectMonitor(qemuProcessQMPPtr proc)
     monConfig.data.nix.listen = false;
 
     if (!(xmlopt = virDomainXMLOptionNew(NULL, NULL, NULL, NULL, NULL)) ||
-        !(proc->vm = virDomainObjNew(xmlopt)))
+        !(proc->vm = virDomainObjNew(xmlopt)) ||
+        !(proc->vm->def = virDomainDefNew()))
         goto cleanup;
 
     proc->vm->pid = proc->pid;