vnc: add support for listen type 'socket'

vnc: add support for listen type 'socket'

VNC graphics already supports sockets but only via 'socket' attribute.
This patch coverts that attribute into listen type 'socket'.

For backward compatibility we need to handle listen type 'socket' and 'socket'
attribute properly to support old XMLs and new XMLs.  If both are provided they
have to match, if only one of them is provided we need to be able to parse that
configuration too.

To not break migration back to old libvirt if the socket is provided by user we
need to generate migratable XML without the listen element and use only 'socket'
attribute.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>