xen: fix the check of e_pfn in xen_find_pfn_range

On some NUMA system, after dom0 up, we see below warning even if there are
enough pfn ranges that could be used for remapping:
"Unable to find available pfn range, not remapping identity pages"

Fix it to avoid getting a memory region of zero size in xen_find_pfn_range.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>