iotests: Restore stty settings on completion

Executing qemu with a terminal as stdin will temporarily alter stty
settings on that terminal (for example, disabling echo), because of
how we run both the monitor and any multiplexing with guest input.
Normally, qemu restores the original settings on exit; but if an
iotest triggers qemu to abort in the middle, we can be left with
the altered terminal setup.  This can make life very annoying when
debugging an iotest failure (not everyone remembers the trick of
blind-typing 'stty sane' without echo, and some people prefer
terminal settings that are slightly different than the defaults
picked by 'stty sane').

It is possible to avoid qemu corrupting the terminal by not passing
a terminal to qemu's stdin in the first place (as in, use
'./check ... </dev/null'), but that's extra typing to have to
remember.  But running 'exec </dev/null' in the harness seems like
it might be too heavy of a hammer.  So I instead went the the
solution of saving and restoring the stty settings, only when the
harness detects that it is run interactively.

I tested this patch by forcing an allocation failure (I can't
guarantee that this particular limit will work on all setups, but
it shows the idea):
 $ (ulimit -S -v 500000; ./check -qcow2 1)

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