rom/device regions have a ram_addr that is composed of both an I/O handler
(low bits) and RAM region (high bits); but qemu_ram_set_idstr() expects just
a RAM region.  Mask the I/O handler to make it happy.
Tested-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NAvi Kivity <avi@redhat.com>