xen/vtd: create RMRR mapping

xen/vtd: create RMRR mapping

RMRR reserved regions must be setup in the pfn space with an identity
mapping to reported mfn. However existing code has problem to setup
correct mapping when VT-d shares EPT page table, so lead to problem
when assigning devices (e.g GPU) with RMRR reported. So instead, this
patch aims to setup identity mapping in p2m layer, regardless of
whether EPT is shared or not. And we still keep creating VT-d table.

And we also need to introduce a pair of helper to create/clear this
sort of identity mapping as follows:

set_identity_p2m_entry():

If the gfn space is unoccupied, we just set the mapping. If space
is already occupied by desired identity mapping, do nothing.
Otherwise, failure is returned.

clear_identity_p2m_entry():

We just define macro to wrapper guest_physmap_remove_page() with
a returning value as necessary.

CC: Tim Deegan <tim@xen.org>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <jbeulich@suse.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Yang Zhang <yang.z.zhang@intel.com>
CC: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>