Honour current user and role in SELinux label generation

Honour current user and role in SELinux label generation

When generating an SELinux context for a VM from the template
"system_u:system_r:svirt_t:s0", copy the role + user from the
current process instead of the template context. So if the
current process is

   unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

then the VM context ends up as

  unconfined_u:unconfined_r:svirt_t:s0:c386,c703

instead of

   system_u:system_r:svirt_t:s0:c177,c424

Ideally the /etc/selinux/targeted/contexts/virtual_domain_context
file would have just shown the 'svirt_t' type, and not the full
context, but that can't be changed now for compatibility reasons.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>