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 5bdcef13
later re-introduced in commit dc3d2c9f
and then left in when removing the old detection again in
commit 18981877

Remove it to properly error out when libxl was requested but not
detected.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Fixes: 18981877Reviewed-by: NAndrea Bolognani <abologna@redhat.com>