i40e: Disable offline diagnostics if VFs are enabled

Require the user to disable virtual functions before running the device
offline diagnostics.  The offline diagnostics are intended to ensure
basic operation of the device - it is beyond the scope of the diagnostic
test to handle the additional complexity of bringing all the virtual
functions offline and then back online for each test run.

Change-ID: Ic0b854851a09fc85df0c9e82c220e45885457c30
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>