x86/xen: prepare p2m list for memory hotplug

Commit 054954eb ("xen: switch to linear
virtual mapped sparse p2m list") introduced a regression regarding to
memory hotplug for a pv-domain: as the virtual space for the p2m list
is allocated for the to be expected memory size of the domain only,
hotplugged memory above that size will not be usable by the domain.

Correct this by using a configurable size for the p2m list in case of
memory hotplug enabled (default supported memory size is 512 GB for
64 bit domains and 4 GB for 32 bit domains).

Signed-off-by: Juergen Gross <jgross@suse.com>
Cc: <stable@vger.kernel.org> # 3.19+
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>