qemu-iotests: add option to show qemu binary logs on stdout

qemu-iotests: add option to show qemu binary logs on stdout

Using the flag -p, allow the qemu binary to print to stdout.

Also create the common function _close_qemu_log_file() to
avoid accessing machine.py private fields directly and have
duplicate code.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210809090114.64834-16-eesposit@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>