range: add Range structure

range: add Range structure

Sometimes we need to pass ranges around, add a
handy structure for this purpose.

Note: memory.c defines its own concept of AddrRange structure for
working with 128 addresses.  It's necessary there for doing range math.
This is not needed for most users: struct Range is
much simpler, and is only used for passing the range around.

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>