x86/xen: export xen_alloc_p2m_entry()

x86/xen: export xen_alloc_p2m_entry()

Rename alloc_p2m() to xen_alloc_p2m_entry() and export it.

This is useful for ensuring that a p2m entry is allocated (i.e., not a
shared missing or identity entry) so that subsequent set_phys_to_machine()
calls will require no further allocations.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
v3:
- Make xen_alloc_p2m_entry() a nop on auto-xlate guests.

(cherry picked from commit 8edfcf882eb91ec9028c7334f90f6ef3db5b0fcf)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>