schemas: Allow all generic elements and attributes for all interfaces

schemas: Allow all generic elements and attributes for all interfaces

There are some interface types (notably 'server' and 'client')
which instead of allowing the default set of elements and
attributes (like the rest do), try to enumerate only the elements
they know of. This way it's, however, easy to miss something. For
instance, the <address/> element was not mentioned at all. This
resulted in a strange behavior: when such interface was added
into XML, the address was automatically generated by parsing
code. Later, the formatted XML hasn't passed the RNG schema. This
became more visible once we've turned on the XML validation on
domain XML changes: appending an empty line at the end of
formatted XML (to trick virsh think the XML had changed) made
libvirt to refuse the very same XML it formatted.

Instead of trying to find each element and attribute we are
missing in the schema, lets just allow all the elements and
attributes like we're doing that for the rest of types. It's no
harm if the schema is wider than our parser allows.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>