qemu: snapshot: Don't overwrite existing errors when thawing filesystems

If we are attempting to thaw the filesystems on error, the code would
overwrite the error code that caused the snapshot to fail with the error
of thawing the filesystem. Since the thawing function allows control of
error reporting behavior we can use this feature.

src/qemu/qemu_driver.c

@@ -14696,7 +14696,7 @@ qemuDomainSnapshotCreateActiveExternal(virConnectPtr conn,
     }
 
     if (thaw != 0 &&
-        qemuDomainSnapshotFSThaw(driver, vm, thaw > 0) < 0) {
+        qemuDomainSnapshotFSThaw(driver, vm, ret == 0 && thaw > 0) < 0) {
         /* helper reported the error, if it was needed */
         if (thaw > 0)
             ret = -1;