conf: Fix even implicit labels

conf: Fix even implicit labels

https://bugzilla.redhat.com/show_bug.cgi?id=1027096#c8

There are two ways in which security model can make it way into
<seclabel/>. One is as the @model attribute, the second one is
via security_driver knob in qemu.conf. Then, while parsing
<seclabel/> several checks and fix ups of old, stale combinations
are performed. However, iff @model is specified. They are not
done in the latter case. So it's still possible to feed libvirt
with senseless combinations (if qemu.conf is adjusted correctly).

One example of a seclabel that needs some adjustment (in case
security_driver=none in qemu.conf) is:

    <seclabel type='dynamic' relabel='yes'/>

The fixup code is copied from virSecurityLabelDefParseXML
(covering the former case) into virSecurityLabelDefsParseXML
(which handles the latter case).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>