qemu: free migEvent in qemuMigrationCapsCheck

We allocate a temporary bitmap but never free it.

Introduced by <commit 1f8a1a9>:
    qemu: Do not use qemuMonitorSetMigrationCapability

Signed-off-by: Ján Tomko <jtomko@redhat.com>

src/qemu/qemu_migration_params.c

@@ -1343,6 +1343,7 @@ qemuMigrationCapsCheck(virQEMUDriverPtr driver,
     ret = 0;
 
  cleanup:
+    virBitmapFree(migEvent);
     virJSONValueFree(json);
     virStringListFree(caps);
     return ret;