virt: Remove all guest sockets on cleanup

Previously vdsm was cleaning up the guest socket files, but when
introducing the symlinks we started leaving behind the real sockets
and just removed the symlinks.
This patch fixes this by first removing the file the symlink points
to and then removes the symlink.

Change-Id: Iaf3bfbd0eedcb6486cbde685d7b6ee64e8dc8d8c
Bug-Url: https://bugzilla.redhat.com/1052097
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26278
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>