snapshot_conf: use alternate domain definition in virDomainSnapshotDefAssignExternalNames

snapshot_conf: use alternate domain definition in virDomainSnapshotDefAssignExternalNames

Commit <ef3f3884a2432958bdd4ea0ce45509d47a91a453> introduced new
argument for virDomainSnapshotAlignDisks() that allows passing alternate
domain definition in case the snapshot parent.dom is NULL.

In case of redefining snapshot it will not hit the part of code that
unconditionally uses parent.dom as there will not be need to generate
default external file names.

It should be still fixed to make it safe. Future external snapshot
revert code will use this to generate default file names and in this
case it would crash.

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