x86/mem_sharing: copy parent VM's hostp2m's max_mapped_pfn during forking

x86/mem_sharing: copy parent VM's hostp2m's max_mapped_pfn during forking

When creating a VM fork copy the parent VM's hostp2m max_mapped_pfn value. Some
toolstack relies on the XENMEM_maximum_gpfn value to establish the maximum
addressable physical memory in the VM and for forks that have not yet been
unpaused that value is not going to reflect the correct max gpfn that's
possible to populate into the p2m. This patch fixes the issue.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>