xen/p2m: Check for auto-xlat when doing mfn_to_local_pfn.

Most of the functions in page.h are prefaced with
	if (xen_feature(XENFEAT_auto_translated_physmap))
		return mfn;

Except the mfn_to_local_pfn. At a first sight, the function
should work without this patch - as the 'mfn_to_mfn' has
a similar check. But there are no such check in the
'get_phys_to_machine' function - so we would crash in there.

This fixes it by following the convention of having the
check for auto-xlat in these static functions.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>