virttest.kvm_virtio_port: New features for GuestWorker

This patch adds new features to GuestWorker:
* read_nonblocking - read-out the output of guest script (used when
                     the test fails and we want to see the output)
* ThSendCheck - adds reduced_set option (use only A-Z, simplifies debuging
* ThRecvCheck - adds debug mode. This mode executes completely different
                loop. It's better for debuging purposes. Not all
                functionalities are kept so you should know when you want
                to use it.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>