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 <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>