drivers: xen: add DMOP hypercall wrappers

Add wrappers for following XEN_DMOP_* hypercalls.
These enables Xen device model control path:
dm_op provides operations to create/manage the ioreq server
so guest MMIO accesses are trapped and handled by the hypervisor.
These are guarded by CONFIG_XEN_DMOP.

- dmop
  - dmop_create_ioreq_server
    XEN_DMOP_create_ioreq_server
  - dmop_map_io_range_to_ioreq_server
    XEN_DMOP_map_io_range_to_ioreq_server
  - dmop_set_ioreq_server_state
    XEN_DMOP_set_ioreq_server_state
  - dmop_nr_vcpus
    XEN_DMOP_nr_vcpus
  - dmop_set_irq_level:
    XEN_DMOP_set_irq_level

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>