minios: Fix xfree() bug.
minios: Fix xfree() bug.

It has to check first if the memory to free is so big as to be freed
directly by free_pages. mini-os domains crash without this patch if
vfb is misconfigured.

Signed-off-by: INAKOSHI Hiroya <10203b031b601c49b4697d8dc6e0ac3ae1886589@jp.fujitsu.com>

rowhit/xen-minios,rowhit/xen-minios,rowhit/xen-minios