x86/mmcfg: add handlers for the PVH Dom0 MMCFG areas

x86/mmcfg: add handlers for the PVH Dom0 MMCFG areas

Introduce a set of handlers for the accesses to the MMCFG areas. Those
areas are setup based on the contents of the hardware MMCFG tables,
and the list of handled MMCFG areas is stored inside of the hvm_domain
struct.

The read/writes are forwarded to the generic vpci handlers once the
address is decoded in order to obtain the device and register the
guest is trying to access.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>