Change the default libvirt remote user to libvirt-remote

Change the default libvirt remote user to libvirt-remote

It is possible to communicate per ssh via a socket with libvirt. It is
not a good idea to do this as root, so the remote user is now
libvirt-remote. Only this user or users in the group libvirt-remote can
communicate with the socket.
The user libvirt-remote is created without a password. The users have to
set a password for this user after installation.

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>