qemuSnapshotCreateActiveExternal: Don't thaw filesystems when freeze fails

qemuSnapshotCreateActiveExternal: Don't thaw filesystems when freeze fails

If we didn't freeze any filesystems we should not even attempt thawing
them. Additionally 'guest-fsfreeze-freeze' fails if the filesystems are
already frozen, where thawing them may break users data integrity if
they used VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE accidentally after an
explicit virDomainFSFreeze and the next snapshot without that flag would
be taken with already thawed filesystems.

This effectively reverts 7c736bab06479ccec59df69fb79a5c06d112d8fb .
Libvirt nowadays checks whether the guest agent is connected and pings
it before issuing an command so it's very unlikely that we'd end up in a
situation where qemuSnapshotCreateActiveExternal froze filesystems and
didn't thaw them.

Additionally we now discourage the use of
VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE since users have better control if
they freeze the FS themselves.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>