libxl: fallback to lib probe if pkgconfig file not found

libxl: fallback to lib probe if pkgconfig file not found

With the assumption that all Xen >= 4.6 contains a pkgconfig file for
libxenlight, commit 5bdcef13 dropped the fallback check to probe
libxenlight with LIBVIRT_CHECK_LIB. At the time it was not known that
the various Xen pkgconfig files are in the -runtime package in Fedora,
instead of the traditional -devel package. This bug [1] was fixed in
Fedora > 28, but until Fedora 28 reaches EOL we'll need to re-introduce
the fallback check.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1629643

Signed-off-by: Jim Fehlig <jfehlig@suse.com>