vhost: rename structs for enabling client mode

vhost: rename structs for enabling client mode

DPDK vhost-user just acts as server so far, so, using a struct named
as "vhost_server" is okay. However, if we add client mode, it doesn't
make sense any more. Here renames it to "vhost_user_socket".

There was no obvious wrong about "connfd_ctx", but I think it's obviously
better to rename it to "vhost_user_connection", as it does represent
a connection, a connection between the backend (DPDK) and the frontend
(QEMU).

Similarly, few more renames are taken, such as "vserver_new_vq_conn"
to "vhost_user_new_connection".

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>