x86/shadow: move OOS flag bit positions

x86/shadow: move OOS flag bit positions

In preparation of reducing struct page_info's shadow_flags field to 16
bits, lower the bit positions used for SHF_out_of_sync and
SHF_oos_may_write.

Instead of also adjusting the open coded use in _get_page_type(),
introduce shadow_prepare_page_type_change() to contain knowledge of the
bit positions to shadow code.

This is part of XSA-280.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>