xen: fix p2m size in dom0 for disabled memory hotplug case

Since commit 9e2369c0 ("xen: add helpers to allocate unpopulated
memory") foreign mappings are using guest physical addresses allocated
via ZONE_DEVICE functionality.

This will result in problems for the case of no balloon memory hotplug
being configured, as the p2m list will only cover the initial memory
size of the domain. Any ZONE_DEVICE allocated address will be outside
the p2m range and thus a mapping can't be established with that memory
address.

Fix that by extending the p2m size for that case. At the same time add
a check for a to be created mapping to be within the p2m limits in
order to detect errors early.

While changing a comment, remove some 32-bit leftovers.

This is XSA-369.

Fixes: 9e2369c0

 ("xen: add helpers to allocate unpopulated memory")
Cc: <stable@vger.kernel.org> # 5.9
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>