m4: libxl: properly fail when libxl is required

m4: libxl: properly fail when libxl is required

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 5bdcef13d13560512c7d6d8c9e8822e456889e0c
later re-introduced in commit dc3d2c9f8c7678a950abedd227b1587ca62335c4
and then left in when removing the old detection again in
commit 18981877d2e20390a79d068861a24e716f8ee422

Remove it to properly error out when libxl was requested but not
detected.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Fixes: 18981877d2e20390a79d068861a24e716f8ee422
Reviewed-by: Andrea Bolognani <abologna@redhat.com>