Support custom 'svirt_tcg_t' context for TCG based guests

Support custom 'svirt_tcg_t' context for TCG based guests

The current SELinux policy only works for KVM guests, since
TCG requires the 'execmem' privilege. There is a 'virt_use_execmem'
boolean to turn this on globally, but that is unpleasant for users.
This changes libvirt to automatically use a new 'svirt_tcg_t'
context for TCG based guests. This obsoletes the previous
boolean tunable and makes things 'just work(tm)'

Since we can't assume we run with new enough policy, I also
make us log a warning message (once only) if we find the policy
lacks support. In this case we fallback to the normal label and
expect users to set the boolean tunable

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 77d3a8097480e388f1ce3129fe530f235b05f93b)