iotests/055: Use cache.no-flush for vmdk target

iotests/055: Use cache.no-flush for vmdk target

055 uses the backup block job to create a compressed backup of an
$IMGFMT image with both qcow2 and vmdk targets. However, cluster
allocation in vmdk is very slow because it flushes the image file after
each L2 update.

There is no reason why we need this level of safety in this test, so
let's disable flushes for vmdk. For the blockdev-backup tests this is
achieved by simply adding the cache.no-flush=on to the drive_add() for
the target. For drive-backup, the caching flags are copied from the
source node, so we'll also add the flag to the source node, even though
it is not vmdk.

This can make the test run significantly faster (though it doesn't make
a difference on tmpfs). In my usual setup it goes from ~45s to ~15s.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200505064618.16267-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>