qemuDomainSnapshotForEachQcow2: Pass in 'def' rather than selecting it internally

qemuDomainSnapshotForEachQcow2: Pass in 'def' rather than selecting it internally

In some cases such as when creating an internal inactive snapshot we
know that the domain definition in the snapshot is equivalent to the
current definition. Additionally we set up the current definition for
the snapshotting but not the one contained in the snapshot. Thus in some
cases the caller knows better which def to use.

Make qemuDomainSnapshotForEachQcow2 take the definition by the caller
and copy the logic for selecting the definition to callers where we
don't know for sure that the above claim applies.

This fixes internal inactive snapshots when <disk type='volume'> is used
as we translate the pool/vol combo only in the current def.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/97
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>