runtime: better handling of QEMU aborts

Add two changes to run_qemu. The first saves/restores terminal settings.
This solves an annoying loss of terminal echo when QEMU aborts during
a test run. The second ensures we see a message about the abort, because
the "Aborted (core dumped)" message we should see gets eaten. We also
add a message to run()'s failure cases in its exit code processing to
handle signals in general.

Note, the first change is necessary because QEMU modifies the terminal
settings when using '-serial stdio', but calling abort() invokes exit
without first calling qemu_chr_free(serial_hds[0]) to restore them.

(Additionally we fixup the premature failure check to only capture the
 last line, like it says it's doing.)

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrew Jones <drjones@redhat.com>
[Redirected the abort message to stderr and moved it below `stty`.]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>