virttest: Close and re-establish serial console every test

In order to not pickle Expect and ShellSession objects,
that will lead to one fd leak average per test, make
sure they are closed and reopened at each test, making
the average fd leak on virt-tests to be effectively 0.

With this we've came a long way since the work of fixing
resource leakage on virt-test started. There are more
places to work on, let's keep on working.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>