tests/qtest/rtl8139-test: Make the test less verbose by default

tests/qtest/rtl8139-test: Make the test less verbose by default

We are facing the issues that some test logs in the gitlab CI are
too big (and thus cut off). The rtl8139-test is one of the few qtests
that prints many lines of output by default when running with V=1, so
it contributes to this problem. Almost all other qtests are silent
with V=1 and only print debug messages with V=2 and higher. Thus let's
change the rtl8139-test to behave more like the other tests and only
print the debug messages with V=2 (or higher).

Message-Id: <20230215124122.72037-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>