Handle VMX domains correctly across xend restarts

Handle VMX domains correctly across xend restarts

This patch remembers the device model pid across xend restarts and
avoids creating duplicate device model processes.

Also, device models don't inherit socket fds from xend (which are closed
on exec now), which used to prevent xend restarting.

Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
Signed-off-by: Xiaohui Xin <xiaohui.xin@intel.com>
Signed-off-by: Arun Sharma <arun.sharma@intel.com>