xen/flask: Wire up XEN_DOMCTL_{get,set}_paging_mempool_size

xen/flask: Wire up XEN_DOMCTL_{get,set}_paging_mempool_size

These were overlooked in the original patch, and noticed by OSSTest which does
run some Flask tests.

Fixes: 22b20bd98c02 ("xen: Introduce non-broken hypercalls for the paging mempool size")
Suggested-by: Daniel Smith <dpsmith@apertussolutions.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>