tests/libqos: Make generic virtio code independent from global_qtest

The libqos library functions should never depend on global_qtest,
since these functions might be used in tests that track multiple
test states. Pass around a pointer to the QTestState instead.

Message-Id: <20190814195920.32023-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>