test virtio_console: Raise TestFail in case of failure

Some tests (polling, sigio, rmmod, ...) raised VirtioPortException
instead of TestFailure. This might be confusing so let's raise
TestFail + original traceback instead.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
test virtio_console: wait for data in rw_nonblocking_mode test

On heavy loaded VM data might not yet be available, give
the machine second chance in case of Errno 11.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>