iotests: Warn if python subprocess is killed

Currently, if a subprocess of a python test (i.e. qemu-io, qemu-img, or
qemu) receives a signal and is subsequently aborted, this is not logged.

This patch makes python tests always check the exit code of these
subprocesses, and emit a message if they have been killed.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
