This patch make xm reboot/shutdown work for vmx doamin.

This patch make xm reboot/shutdown work for vmx doamin.

xm reboot fails due to two issues:
1. no mechanism to change XendDomainInfo.info to trigger domain reboot
2. ioemu blkif parameter is missing during reboot, thus device model recreation will fail.

This patch fix these issues by
1. introducing a xswatch to monitor the control/shutdown node. once fired, it will change the XendDomainInfo.info to trigger domain reboot
2. saving the ioemu blkif parameter in xen store just like DomU does.

Signed-off-by: Yu Ke <ke.yu@intel.com>