Prepare for multiple address space support by abstracting away the details
of registering a memory range with qemu's flat representation into an
AddressSpace object.

Note operations which are memory specific are not abstracted, since they will
never be called on I/O address spaces anyway.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>