doc: update vhost guide

doc: update vhost guide

Mainly on updating vhost-user part: we now support client mode.
Also refine some words, and add a bit more explanation.

And made an emphatic statement that you are suggested to use vhost-user
instead of vhost-cuse, because we have enhanced vhost-user a lot since
v2.2 (Actually, I doubt there are any people still using vhost-cuse)

[John McNamara: rewords, better formats]
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>