libxl: Use XEN_LIB_DIR to store bootloader from pygrub

libxl: Use XEN_LIB_DIR to store bootloader from pygrub

In osstest, the jobs using pygrub on arm64 on the branch linux-linus
started to fails with:
    [Errno 28] No space left on device
    Error writing temporary copy of ramdisk

This is because /var/run is small when dom0 has only 512MB to work
with, /var/run is only 40MB. The size of both kernel and ramdisk on
this jobs is now about 42MB, so not enough space in /var/run.

So, to avoid writing a big binary in ramfs, we will use /var/lib
instead, like we already do when saving the device model state on
migration.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
master commit: ad89640ad766d3cb6c92fc8b6406ca6bbab44136
master date: 2023-08-08 09:45:20 +0200