sec_manager: Refuse to start domain with unsupported seclabel

sec_manager: Refuse to start domain with unsupported seclabel

https://bugzilla.redhat.com/show_bug.cgi?id=947387

If a user configures a domain to use a seclabel of a specific type,
but the appropriate driver is not accessible, we should refuse to
start the domain. For instance, if user requires selinux, but it is
either non present in the system, or is just disabled, we should not
start the domain. Moreover, since we are touching only those labels we
have a security driver for, the other labels may confuse libvirt when
reconnecting to a domain on libvirtd restart. In our selinux example,
when starting up a domain, missing security label is okay, as we
auto-generate one. But later, when libvirt is re-connecting to a live
qemu instance, we parse a state XML, where security label is required
and it is an error if missing:

  error : virSecurityLabelDefParseXML:3228 : XML error: security label
  is missing

This results in a qemu process left behind without any libvirt control.