virttest.qemu_vm: make test loop run correctly when using openvswitch

Currently, when close tap device, it's can not automatically cleaned
from openvswitch-db, sinece openvswitch have it's own userspace db, and
that ovsdb is considered to be only modified by the user. when
qemu exit by error, the ovs will never delete the port from it's
db, even kernel have destroied the port, as a result, when we run a
loop using ovs, it alway raise error for the port already exist.

This patch just add a workaround, make the test loop using ovs
can run normally.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>