qemu.tests.virtio_console: Replace ranges for xranges in loops

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.virtio_console: test_perf fixes

* clear the exitevent before each iteration
* raise exception in case no data were transfered
* prevent infinite loop when data are not read-out

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #40 from ldoktor/virtio_console

qemu.tests.virtio_console: test_perf fixes