sysbus_init_mmio_cb() uses the destructive IO_MEM_UNASSIGNED to remove a
region.  Provide an alternative that calls an unmap callback, so the removal
may be done non-destructively.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>