security: Refactor virSecurityManagerGenLabel

security: Refactor virSecurityManagerGenLabel

if (mgr == NULL || mgr->drv == NULL)
    return ret;

This check isn't really necessary, security manager cannot be a NULL
pointer as it is either selinux (by default) or 'none', if no other driver is
set in the config. Even with no config file driver name yields 'none'.

The other hunk checks for domain's security model validity, but we should
also check devices' security model as well, therefore this hunk is moved into
a separate function which is called by virSecurityManagerCheckAllLabel that
checks both the domain's security model and devices' security model.

https://bugzilla.redhat.com/show_bug.cgi?id=1165485
Signed-off-by: Ján Tomko <jtomko@redhat.com>