virsh: Make --type argument of detach-interface optional

virsh: Make --type argument of detach-interface optional

The detach-interface virsh command requires domain (obviously)
and --type to identify <interface/>. Optionally, --mac can be
provided to chose from multiple interfaces. Well, that renders
--type argument redundant. I mean, if there are but unique MACs
within domain XML, then interface type is implied. If there are
duplicate MACs then --type can help to differentiate, though at
that point detach-device seems like a better fit.

Long story short, make --type optional.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>