exec.c: Convert subpage memory ops to _with_attrs

exec.c: Convert subpage memory ops to _with_attrs

Convert the subpage memory ops to _with_attrs; this will allow
us to pass the attributes through to the underlying access
functions. (Nothing uses the attributes yet.)

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Edgar E. Iglesias <[email protected]>