vhost_user: fix bugs and refine message defintion
vhost_user: fix bugs and refine message defintion

Fix two bugs:
1) in VhostUserConfigFlags definition
2) in validating VhostUserConfig

Refine vhost-user message definition by:
1) validate offset of VhostUserConfig
2) better documentation
3) more unit tests

Signed-off-by: Liu Jiang <aaf6961e30ae80fd3a577c4cb6742ab6a1ad13eb@linux.alibaba.com>

rust-vmm/vhost