src: reject empty string for 'dname' in migrate APIs

src: reject empty string for 'dname' in migrate APIs

A domain name is expected to be non-empty, and we validate this when
parsing XML, or accepting a new name during renames. We fail to
enforce this property, however, when performing a migration. This
was discovered when a user complained about inaccessible VMs after
migrating with the Rust APIs which mistakenly hardcoded 'dname' to
the empty string.

Fixes: https://gitlab.com/libvirt/libvirt-rust/-/issues/11
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>