phys_page_find_alloc: Use correct initial region_offset.

This fixes a common bug with initial region_offset value.
Usually, the pages are re-assigned afterwards, so the bug
has a very small effect on regular QEMU use flows.

Signed-off-by: Alex Rozenman <Alex_Rozenman@mentor.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>