igb: add vfs_allocated_count as placeholder for number of vfs

This is the first step in supporting sr-iov.  The vf_allocated_count value
will be 0 until we actually have vfs present.  In the meantime it
represents an offset value for the start of the queues.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>