x86/xen: compactly store large identity ranges in the p2m

Large (multi-GB) identity ranges currently require a unique middle page
(filled with p2m_identity entries) per 1 GB region.

Similar to the common p2m_mid_missing middle page for large missing
regions, introduce a p2m_mid_identity page (filled with p2m_identity
entries) which can be used instead.

set_phys_range_identity() thus only needs to allocate new middle pages
at the beginning and end of the range.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>