libxl: use xenlight pkgconfig file if present

libxl: use xenlight pkgconfig file if present

xen.git commit babeca32 added a pkgconfig file for libxenlight,
allowing libxl apps to determine the location of Xen binaries
such as firmware blobs, device emulator, etc.

This patch adds support for xenlight.pc in the libxl driver, falling
back to the previous configure logic if not found.  It introduces
LIBXL_FIRMWARE_DIR and LIBXL_EXECBIN_DIR to define the firmware and
libexec_bin locations.  If xenlight.pc does not exist, the defines
are set to the current hardcoded paths.  The capabilities'
<emulator> and <loader> elements are updated to use the paths.

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