virtio-net: fill only rx queues which are being used

Due to MQ support we may allocate a whole bunch of rx queues but
never use them. With this patch we'll safe the space used by
the receive buffers until they are actually in use:

sh-4.2# free -h
             total       used       free     shared    buffers     cached
Mem:          490M        35M       455M         0B         0B       4.1M
-/+ buffers/cache:        31M       459M
Swap:           0B         0B         0B
sh-4.2# ethtool -L eth0 combined 8
sh-4.2# free -h
             total       used       free     shared    buffers     cached
Mem:          490M       162M       327M         0B         0B       4.1M
-/+ buffers/cache:       158M       331M
Swap:           0B         0B         0B

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>