iotests: Add test for external image truncation

iotests: Add test for external image truncation

It should not be happening, but it is possible to truncate an image
outside of qemu while qemu is running (or any of the qemu tools using
the block layer. raw_co_get_block_status() should not break then.

While touching this test, replace the existing "truncate" invocation by
"$QEMU_IMG convert -f raw".

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1414148280-17949-4-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>