The current default of 16 buffers for the control vq is too small.  We
can get more entries in there, for example when asking the guest to add
max. allowed ports.

Note: a more robust solution would involve some kind of event queueing
in host to guarantee no event loss. Added a TODO to look into
this later.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>