memory: accept mismatching sizes in memory_region_access_valid

The memory API is able to use smaller/wider accesses than requested,
match that in memory_region_access_valid.  Of course, the accepts
callback is still free to reject those accesses.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>