lxcCapsInit: Allocate primary security driver unconditionally

Currently, if the primary security driver is 'none', we skip
initializing caps->host.secModels. This means, later, when LXC domain
XML is parsed and <seclabel type='none'/> is found (see
virSecurityLabelDefsParseXML), the model name is not copied to the
seclabel. This leads to subsequent crash in virSecurityManagerGenLabel
where we call STREQ() over the model (note, that we are expecting model
to be !NULL).
(cherry picked from commit 37d96498c6a9c3030bfad7dfbd273af5fbdd1845)

Conflicts:
	src/lxc/lxc_conf.c
virSecurityManagerGenLabel: Skip seclabels without model

While generating seclabels, we check the seclabel stack if required
driver is in the stack. If not, an error is returned. However, it is
possible for a seclabel to not have any model set (happens with LXC
domains that have just <seclabel type='none'>). If that's the case,
we should just skip the iteration instead of calling STREQ(NULL, ...)
and SIGSEGV-ing subsequently.
(cherry picked from commit ba44dd2453d486e9eb8c6204f8d7c31d07007d8f)