faqemu hook: move memory limitation to config

*** WARNING: this patch changes default behaviour for faqemu, increasing
memory usage to what is supplied in vmCreate call unless changed in
/etc/vdsm/vdsm.conf ***

This patch removes memory constraints of faqemu, moving one global
constraint to configuration. Setting this to '0' causes memory element not
be touched by the hook, using nonzero value sets the memory limit.

Usage of > 20 MB memory is required
when running faqemu with system kernel/initramfs combo.

Change-Id: I0ffeaf7926dac2a0db0bdfb21416919452718531
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28320
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>