qemu-iotests: s390x: fix test 051

qemu-iotests: s390x: fix test 051

The tests for ide device should only be tested for the pc
platform.
Set device_id to "drive0", and replace every "-drive file..."
by "-drive file=...,if=none,id=$device_id", then x86 and s390x
can get the common output in the test of "Snapshot mode".
Warning message expected for s390x when drive without device.
A x86 platform specific output file is also needed.

Reviewed-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Bo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1449136891-26850-3-git-send-email-tubo@linux.vnet.ibm.com
Signed-off-by: Max Reitz <mreitz@redhat.com>