if instanceId is NULL

When virNetDevVPortProfileGetStatus() was called with instanceId =
NULL (which is the case for all DISASSOCIATE requests in 802.1Qbh) it
would log the following error:

   Could not find netlink response with expected parameters

even though the disassociate had been successfully completely. Then,
due to the fortunate coincidence of status having been initialized to
0 and then not changed when the "failure" was encountered, it would
still return a status of 0 (PORT_VDP_RESPONSE_SUCCESS), so the caller
would assume a successful operation.

This would result in a spurious log message though, and would fill in
LastErrorMessage, so that the API would return that error if it
happened during cleanup from some other error. That, in turn, would
lead to an incorrect supposition that the response to the port profile
disassociate was the cause of the failure.

During debugging, I noticed that the VF in question usually had *no
uuid* associated with it (big surprise)by the time the disassociate
completed, so the solution is *not* to send the previous instanceId
down.

This patch fixes virNetDevVPortProfileGetStatus() to only check the
VF's uuid in the status if it was given an instanceId to check against
when originally called. Otherwise it only checks that the particular
VF is present (it will be).

This does cause a slight difference in behavior - rather than
returning with status unchanged (and thus always 0) it will actually
get the IFLA_PORT_RESPONSE. This could lead to revelation of error
conditions we were previously ignoring. Or not. So far "not".