libqtest: Improve error reporting for bad read from QEMU

libqtest: Improve error reporting for bad read from QEMU

When read() from the qtest socket or the QMP socket fails or EOFs, we
report "Broken pipe" and exit(1).  This commonly happens when QEMU
crashes.  It also happens when QEMU refuses to run because the test
passed it bad arguments.  Sadly, we neglect to report either.

Improve this by calling abort() instead of exit(1), so kill_qemu()
runs, and reports how QEMU died.  This improves error reporting to
something like

    /x86_64/device/introspect/list: Broken pipe
    tests/libqtest.c:129: kill_qemu() detected QEMU death from signal 6 (Aborted) (dumped core)

Three exit() remain in libqtest.c:

* In qmp_response(), when we can't parse a QMP reply read from the QMP
  socket.  Change to abort() for consistency.

* In qtest_qemu_binary(), when QTEST_QEMU_BINARY isn't in the
  environment.  This can only happen before we start QEMU.  Leave
  alone.

* In qtest_init_without_qmp_handshake(), when the fork()ed child fails
  to execlp().  Leave alone.

exit() elsewhere are unlikely due to QEMU dying on us.  If that should
turn out to be wrong, we can move kill_qemu() from @abrt_hooks to
atexit() or something.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180815141945.10457-2-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[Commit message tweaked slightly]