qemu: Fix regression in snapshot-revert

Commit f10562799 introduced a regression: if reverting to a snapshot
fails early (such as when we refuse to revert to an external
snapshot), we lose track of the domain's current snapshot.

Before that patch, we were tracking the notion of the domain's current
snapshot via two means: vm->current_snapshot (which was left untouched
on early exit) and snap->def->current (which only controls what gets
written to XML to remember snapshots across libvirtd restarts).  That
patch was fixing a real bug: if a revert operation failed early, later
questions from the same libvirtd did not see any change to the current
snapsthot, but restarting libvirtd would now claim there is no current
snapshot.  But it fixed it in the wrong direction, in that the current
snapshot was forgotten unconditionally, rather than only when the
snapshot to revert to has a chance of being useful.

It didn't help that the code after that patch had two separate spots
clearing the old notion of the current snapshot - one after
determining the snapshot to revert to was viable, the other
unconditionally on all failure exit paths.  At any rate, the fix is
simple: drop the unconditional cleanup on error paths, and rely only
on the normal cleanup after early checks.

Sadly, it is not possible to test this bug in the existing
tests/virsh-snapshot, as the test driver does not have the same
prohibition against reverting to an external snapshot as the qemu
driver.

See: https://bugzilla.redhat.com/1738747
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190909205242.15406-1-eblake@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>

src/qemu/qemu_driver.c

@@ -16941,8 +16941,6 @@ qemuDomainRevertToSnapshot(virDomainSnapshotPtr snapshot,
             virDomainSnapshotSetCurrent(vm->snapshots, NULL);
             ret = -1;
         }
-    } else if (snap) {
-        virDomainSnapshotSetCurrent(vm->snapshots, NULL);
     }
     if (ret == 0 && config && vm->persistent &&
         !(ret = virDomainSaveConfig(cfg->configDir, driver->caps,