As pointed out by Ján Tomko, "no_memory seems suspicious in the times of
abort()".

As libvirt decided to take the path to not report OOM and simply abort
when it happens, let's get rid of the no_memory labels and simplify the
code around them.

Mind that virfirewall.c was not touched and still contains no_memory
labels. The reason those are left behind, at least for now, is because
the conversion seems to be slightly more complicated than the rest, as
some other places are relying on firewall->err being set to ENOMEM.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NFabiano Fidêncio <fidencio@redhat.com>