qemu: restore: Fix restoring of VM when the restore hook returns empty XML

The documentation for the restore hook states that returning an empty
XML is equivalent with copying the input. There was a bug in the code
checking the returned string by checking the string instead of the
contents. Use the new helper to check if the string is empty.

(cherry picked from commit e3867799)

src/qemu/qemu_driver.c

@@ -5720,7 +5720,7 @@ qemuDomainRestoreFlags(virConnectPtr conn,
                                    &xmlout)) < 0)
             goto cleanup;
 
-        if (hookret == 0 && xmlout) {
+        if (hookret == 0 && !virStringIsEmpty(xmlout)) {
             VIR_DEBUG("Using hook-filtered domain XML: %s", xmlout);
             hook_taint = true;
             newxml = xmlout;
@@ -5936,7 +5936,7 @@ qemuDomainObjRestore(virConnectPtr conn,
                                    NULL, xml, &xmlout)) < 0)
             goto cleanup;
 
-        if (hookret == 0 && xmlout) {
+        if (hookret == 0 && !virStringIsEmpty(xmlout)) {
             virDomainDefPtr tmp;
 
             VIR_DEBUG("Using hook-filtered domain XML: %s", xmlout);