turn err into errx in lguest call sites

These two callsites should really be errx instead of err, since there is
no errno associated with them in the moment they are issued.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Glauber de Oliveira Costa <gcosta@redhat.com>