This commit adds the unix socket URL support to 'domdisplay' command.
Before, even if an user was using unix socket to define a spice graphics,
the command 'domdisplay' showed that the settings were not supported. Now,
the command shows the proper URL: spice+unix://foo/bar.sock.

Settings:
<graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1'>
  <listen type='address' address='127.0.0.1'/>
</graphics>
<graphics type='spice'>
  <listen type='socket' socket='/tmp/spice.sock'/>
</graphics>

Before:
virsh # domdisplay --all Windows7
vnc://127.0.0.1:0

After:
virsh # domdisplay --all Windows7
vnc://127.0.0.1:0
spice+unix:///tmp/spice.sock

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1336720Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>