qemu: Fix possible memory leak in migration param processing

If virJSONValueArraySize(caps) <= 0, then we will still need to
virJSONValueFree(caps) because qemuMonitorSetMigrationCapabilities
won't consume it.

Found by Coverity

Signed-off-by: John Ferlan <jferlan@redhat.com>

src/qemu/qemu_migration_params.c

@@ -771,6 +771,7 @@ qemuMigrationParamsApply(virQEMUDriverPtr driver,
         migParams->params[xbzrle].set = true;
 
     virJSONValueFree(params);
+    virJSONValueFree(caps);
 
     return ret;
 }