hw/loader: split out load_image_gzipped_buffer()

In the next patch we'd like to reuse the image decompression facility
without installing the output as a ROM at a specific guest-phys address.

In addition, expose LOAD_IMAGE_MAX_GUNZIP_BYTES, because that's a
straightforward "max_sz" argument for the new load_image_gzipped_buffer().

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1419250305-31062-10-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
