loader: Add load_image_mr() to load ROM image to a MemoryRegion

Add a new function load_image_mr(), which behaves like
load_image_targphys() except that it loads the ROM image to
a specified MemoryRegion rather than to a specified physical
address. This is useful when a ROM blob needs to be loaded
to a particular flash or ROM device but the address of that
device in the machine's address space is not known. (For
instance, ROMs in devices, or ROMs which might exist in
a different address space to the system address space.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1455288361-30117-3-git-send-email-peter.maydell@linaro.org
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>