memory: check memory region endianness, not target's

When combining multiple accesses into a single value, we need to do so
in the device's desired endianness.  The target endianness does not have
any influence.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Message-id: 1374501278-31549-28-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
