xen/pvh: place the trampoline at page 0x1

xen/pvh: place the trampoline at page 0x1

Since PVH guest jump straight into trampoline_setup trampoline_phys is
not initialized, thus the trampoline is relocated to address 0.

This works, but has the undesirable effect of having VA 0 mapped to
MFN 0, which means NULL pointed dereferences no longer trigger a page
fault.

In order to solve this, place the trampoline at page 0x1 and reserve
the memory used by it.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>