If SELinux is compiled into libvirt but it is disabled on the host,
libvirtd logs:

  error : virIdentityGetSystem:173 : Unable to lookup SELinux process
  context: Invalid argument

on each and every client connection.

Use is_selinux_enabled() to skip retrieval of the process's SELinux
context if SELinux is disabled.
Signed-off-by: NMichael Chapman <mike@very.puzzling.org>