We specify "true" as the fail-action for LIBVIRT_CHECK_PKG.
This was used when we had a fallback to non-pkg-config detection,
then removed in commit 5bdcef13d1
later re-introduced in commit dc3d2c9f8c
and then left in when removing the old detection again in
commit 18981877d2
Remove it to properly error out when libxl was requested but not
detected.
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Fixes: 18981877d2
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
