Since segments need to be handled slightly differently when fetching
instructions, we add a __linearize helper that accepts a new 'fetch' boolean.

[avi: fix oops caused by wrong segmented_address initialization order]
Signed-off-by: NNelson Elhage <nelhage@ksplice.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>