fix buffer allocation for sparse jumbo frames in vhost

fix buffer allocation for sparse jumbo frames in vhost

A bug was reported where a jumbo packet would stay in vhost
queue forever or until a large enough number of other packets
arrived in the queue too.

This is due to a bug in vhost input node buffer allocation.

The fix is to make sure that vhost always allocates at least
enough buffers for one single big packet. '40' is used to
account for 65kB frames.

Change-Id: I1d293028854165083e30cd798fab9d4140230b78
Signed-off-by: Pierre Pfister <ppfister@cisco.com>