hw/pci-host/q35: Inline sysbus_add_io()

hw/pci-host/q35: Inline sysbus_add_io()

sysbus_add_io() just wraps memory_region_add_subregion() while also
obscuring where the memory is attached. So use
memory_region_add_subregion() directly and attach it to the existing
memory region s->mch.address_space_io which is set as an alias to
get_system_io() by the q35 machine.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230213162004.2797-3-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>