vhost_user: Fix & comment "feature cache"
vhost_user: Fix & comment "feature cache"

Added a long description of the "feature cache" and how it supports
reconnecting to a running QEMU.

Added a restriction so that the cache is only considered for the first
feature negotiation performed by a vhost-user app instance. This is to
avoid using the cached features when renegotiating with a guest e.g.
when the guest has switched to a new driver. Not yet tested.

wingo/snabbswitch,mixflowtech/logsensor,eugeneia/snabb,alexandergall/snabbswitch,wingo/snabb,dpino/snabb,alexandergall/snabbswitch,dpino/snabbswitch,SnabbCo/snabbswitch,Igalia/snabb,Igalia/snabb,Igalia/snabb,kbara/snabb,wingo/snabbswitch,heryii/snabb,eugeneia/snabb,Igalia/snabb,snabbco/snabb,snabbco/snabb,dpino/snabb,alexandergall/snabbswitch,dpino/snabbswitch,heryii/snabb,Igalia/snabbswitch,Igalia/snabb,wingo/snabb,mixflowtech/logsensor,eugeneia/snabb,snabbco/snabb,eugeneia/snabbswitch,Igalia/snabbswitch,alexandergall/snabbswitch,dpino/snabb,SnabbCo/snabbswitch,dpino/snabb,dpino/snabb,eugeneia/snabbswitch,kbara/snabb,Igalia/snabbswitch,dpino/snabbswitch,eugeneia/snabbswitch,wingo/snabb,SnabbCo/snabbswitch,Igalia/snabb,heryii/snabb,Igalia/snabb,wingo/snabb,kbara/snabb,snabbco/snabb,snabbco/snabb,dpino/snabb,Igalia/snabb,wingo/snabbswitch,Igalia/snabbswitch,snabbco/snabb,SnabbCo/snabbswitch,wingo/snabb,wingo/snabb,kbara/snabb,eugeneia/snabb,kbara/snabb,eugeneia/snabbswitch,snabbco/snabb,eugeneia/snabb,eugeneia/snabb,eugeneia/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,dpino/snabb,Igalia/snabbswitch,heryii/snabb,heryii/snabb,mixflowtech/logsensor,heryii/snabb,mixflowtech/logsensor,dpino/snabbswitch,mixflowtech/logsensor,alexandergall/snabbswitch,snabbco/snabb,eugeneia/snabb,alexandergall/snabbswitch,wingo/snabbswitch,kbara/snabb