qga: Move HW address getting to a separate function

In the next patch FreeBSD support for guest-network-get-interfaces will be
added. Previously move Linux-specific code of HW address getting to a
separate functions and add a dumb function to commands-bsd.c.

Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>