openvswitch: Delete port if it exists while adding a new one

openvswitch: Delete port if it exists while adding a new one

If the openvswitch service is stopped, and is followed by destroying a
VM, the openvswitch bridge translates into a state where it doesn't
recover the port configuration. While it successfully fetches data
from the internal DB, since the corresponding virtual interface does
not exists anymore the whole recovery process fails leaving restarted
VM with inability to connect to the bridge. The following set of
commands will trigger the problem:

virsh start vm
service openvswitch-switch stop
virsh destroy vm
service openvswitch-switch start
virsh start vm

Signed-off-by: Chunhe Li <lichunhe@huawei.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>