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>