vhost: export queue free entries

vhost: export queue free entries

The new API rte_vhost_avail_entries() is actually a rename of
rte_vring_available_entries(), with the "vring" to "vhost" name
change to keep the consistency of other vhost exported APIs.

This change could let us avoid the dependency of "virtio_net"
struct, to prepare for the ABI refactoring.

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>