vhost: increase number of async IO vectors

vhost: increase number of async IO vectors

This patch increases the number of IO vectors for the
asynchronous data path from 512 to 2048. It has been
reported during testing the starvation of IO vectors
during iperf benchmark with 64KB packet size.

As there are no direct relationship between
VHOST_MAX_ASYNC_VEC and BUF_VECTOR_MAX, this patch also
assign VHOST_MAX_ASYNC_VEC value directly instead of being
a multiple of BUF_VECTOR_MAX.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Jiayu Hu <jiayu.hu@intel.com>