libvirt: set path to pkcheck

The libvirt build would fail whenever x11 was in DISTRO_FEATURES and the host
didn't have polkit installed (either no polkit/policykit at all, or just old
policykit).

Ensure that libvirt uses polkit rather than policykit, whether the host has
pkcheck installed or not, and ensure the path is correct per our config.

JIRA: SB-3053

Signed-off-by: Christopher Larson <kergoth@gmail.com>