VMX: allocate APIC access page from domain heap

VMX: allocate APIC access page from domain heap

... since we don't need its virtual address anywhere (it's a
placeholder page only after all). For this to work (and possibly be
done elsewhere too) share_xen_page_with_guest() needs to mark pages
handed to it as Xen heap ones.

To be on the safe side, also explicitly clear the page (not having done
so was okay due to the XSA-100 fix, but is still a latent bug since we
don't formally guarantee allocations to come out zeroed, and in fact
this property may disappear again as soon as the asynchronous runtime
scrubbing patches arrive).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>