If mgmt apps on top of libvirt want to make a decision on the
backend type for <interface type='user'/> (e.g. whether past is
supported) we currently offer them no way to learn this fact.
Domain capabilities were invented exactly for this reason. Report
supported net backend types there.

Now, because of backwards compatibility, specifying no backend
type (which translates to VIR_DOMAIN_NET_BACKEND_DEFAULT) means
"use hyperviosr's builtin SLIRP". That behaviour can not be
changed. But it may happen that the hypervisor has no support for
SLIRP. So we have to report it.

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