ixgbe: Use VMDq offset to indicate the default pool

This change makes it so that we can use the VMDq ring feature offset value
to determine the default pool instead of using num_vfs.  The reason for
this change is to avoid issues should we fail to allocate vfinfo but have
pre-existing VFs.  What should happen in this case is that num_vfs will go
to 0, but the VMDq offset will contain the location of the first PF pool.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Sibai Li <Sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>