xen: Don't use memory_region_init_ram_nomigrate() in pci_assign_dev_load_option_rom()

xen: Don't use memory_region_init_ram_nomigrate() in pci_assign_dev_load_option_rom()

The xen pci_assign_dev_load_option_rom() currently creates a RAM
memory region with memory_region_init_ram_nomigrate(), and then
manually registers it with vmstate_register_ram(). In fact for
its only callsite, the 'owner' pointer we use for the init call
and the '&dev->qdev' pointer we use for the vmstate_register_ram()
call refer to the same object. Simplify the function to only
take a pointer to the device once instead of twice, and use
memory_region_init_ram() which automatically does the vmstate
register for us.

Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>