Ensure the vm is running when vm operations are called

[gnawux]

part of the fix of hyperhq/hyperd#598 (pod creation rollback), changes include:

• change context lock from Mutex to RWMutex
• move ctx.current access inside lock
• ensure vm operations has the running status check

Signed-off-by: Wang Xu gnawux@gmail.com

[laijs]

LGTM