virDomainSnapshotAlignDisks: Allow alternate domain definition when redefining

virDomainSnapshotAlignDisks: Allow alternate domain definition when redefining

Due to historical reasons we allow users to redefine an existing
snapshot without providing the domain definition which would correspond
to it. In such case we'd use the domain definition from the snapshot
that is being redefined.

To prevent callers from doing complex moving of the domain definition
object back and forth between the snapshot definitions we can add an
argument to virDomainSnapshotAlignDisks which will allow us to pass in
the alternate definition if the one from the snapshot is missing.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>