vhost: fold common message handlers

vhost: fold common message handlers

Due to history reason (that we have 2 vhost implementations), some
messages are handled in two calls: vhost specific implementation
handles it first and then invoke the common one to do another handling.

We have one implementation only now, we could write one method for
each message. Here fold those common handles to corresponding vhost
user handler.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>