x86: add a new p2m type - p2m_mmio_write_dm

x86: add a new p2m type - p2m_mmio_write_dm

A new p2m type, p2m_mmio_write_dm, is added to trap and emulate
the write operations on GPU's page tables. Handling of this new
p2m type are similar with existing p2m_ram_ro in most condition
checks, with only difference on final policy of emulation vs. drop.
For p2m_ram_ro types, write operations will not trigger the device
model, and will be discarded later in __hvm_copy(); while for the
p2m_mmio_write_dm type pages, writes will go to the device model
via ioreq-server.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Wei Ye <wei.ye@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>