ixgbe: Do not run all Diagnostic offline tests when VFs are active

When running the offline diagnostic tests check to see if any VFs are
online.  If so then only run the link test.  This is necessary because
the VFs running in guest VMs aren't aware of when the PF is taken
offline for a diagnostic test.  Also put a message to the system log
telling the system administrator to take the VFs offline manually if
(s)he wants to run a full diagnostic.  Return 1 on each of the tests
not run to alert the user of the condition.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>