vhost: make buffer vector for scatter Rx local

vhost: make buffer vector for scatter Rx local

Array of buf_vector's is just an array for temporary storing information
about available descriptors. It used only locally in virtio_dev_merge_rx()
and there is no reason for that array to be shared.

Fix that by allocating local buf_vec inside virtio_dev_merge_rx().

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: Rich Lane <rich.lane@bigswitch.com>
Acked-by: Rich Lane <rich.lane@bigswitch.com>