vmnet: distinguish client from server in the logging
vmnet: distinguish client from server in the logging

When debugging traces from the test cases it's useful to know whether
a `Vmnet.listen` call is from the client end or the server. This patch
adds a logging prefix and sets this to either `Vmnet.Client` or
`Vmnet.Server`.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit