hmp: fix crash in 'info block -n -v'

The image field in BlockDeviceInfo should never be null, however
bdrv_block_device_info() is not filling it in.

This makes the 'info block -n -v' command crash QEMU.

The proper solution is probably to move the relevant code from
bdrv_query_info() to bdrv_block_device_info(), but since we're too
close to the release for that this simpler workaround solves the
crash.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 1429274688-8115-1-git-send-email-berto@igalia.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
