qed: Provide auxiliary for getting free VF SB

IOV code is very intrusive in its manipulation of the status block
database.
Add a new auxiliary function to allow the PF to find an available unused
status block to configure for a specific VF's MSI-x vector.

Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>