memory: rename 'exec-obsolete.h'

memory: rename 'exec-obsolete.h'

exec-obsolete.h used to hold pre-memory-API functions that were used from
device code prior to the transition to the memory API.  Now that the
transition is complete, the name no longer describes the file.  The
functions still need to be merged better into the memory core, but there's
no danger of anyone using them.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>