qemu-iotests: Fix locking issue in 102

102 truncates a qcow2 file (the raw file) on purpose while a VM is
running.  However, image locking will usually prevent exactly this.

The fact that most people have not noticed until now (I suppose you may
have seen sporadic failures, but not taken them too seriously, like me)
further shows that this truncation is actually not really done
concurrently, but that the VM is still starting up by this point and has
not yet opened the image.  Remedy this by waiting for the monitor shell
to appear before the qemu-img invocation so we know the VM is up.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171129185102.29390-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
