Add a generic loader to QEMU which can be used to load images or set
memory values.

Internally inside QEMU this is a device. It is a strange device that
provides no hardware interface but allows QEMU to monkey patch memory
specified when it is created. To be able to do this it has a reset
callback that does the memory operations.

This device allows the user to monkey patch memory. To be able to do
this it needs a backend to manage the datas, the same as other
memory-related devices. In this case as the backend is so trivial we
have merged it with the frontend instead of creating and maintaining a
seperate backend.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-id: 10f2a9dce5e5e11b6c6d959415b0ad6ee22bcba5.1475195078.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>