libxl, docs: Add per-arch extra default paging memory

libxl, docs: Add per-arch extra default paging memory

This commit adds a per-arch macro `EXTRA_DEFAULT_PAGING_MEM_MB`
to the default paging memory size, in order to cover the p2m
pool for extended regions of a xl-based guest on Arm.

For Arm, the extra default paging memory is 128MB.
For x86, the extra default paging memory is zero, since there
are no extended regions on x86.

Also update the xl.cfg documentation to add Arm documentation
according to code changes.

This is part of CVE-2022-33747 / XSA-409.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>