qemu.tests.virtio_console: Kill immortal background threads

Take care of freezed sender thread everywhere as in 66e657

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virtest.qemu_virtio_port: Disable serial-getty@hvc0.service

SystemD automatically spawns console on /dev/hvc0 if exists.
(even if it's not specified on initrd line). Since guest worker
uses hvc0 as plain pipe we have to mask this service.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.virtio_console: Raise TestError when S3/S4 cmd is missing

Fail with error.TestError in case suspend or hibernate commands are
missing.

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