loader: support for unmapped ROM blobs

loader: support for unmapped ROM blobs

Support ROM blobs not mapped into guest memory:
same as ROM files really but use caller's buffer.

Support invoking callback on access and
return memory pointer making it easier
for caller to update memory if necessary.

Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>