Implement support for "xl shutdown"

[mato]

Currently there is no support for "xl shutdown" and the only way to shutdown a rumprun-xen domU is to kill it hard with "xl destroy". We should consider how to implement this, hopefully without opening the Pandora's box with "SIGTERM" written on it.

[anttikantee]

As a first attempt, just set a "shutdown" flag and catch threads when they enter the scheduler. It's not 100% emulation, as explicit sigterm handlers will not be called, but I think just making sure the rump kernel processes exit before calling reboot will already do wonders (TCP connections will be closed, etc.).