dm: vm_event: create vm_event thread

This patch creates a thread for vm_event delivery. The thread uses epoll
to poll event notifications, then read out the msg data queued in sbuf.
An event handler is called upon success receiving. Both HV and DM event
sources share the same process.

Also vm_event tx API for DM event source is added in this patch.

Tracked-On: #8547
Signed-off-by: Wu Zhou <wu.zhou@intel.com>
Reviewed-by: Jian Jun Chen <jian.jun.chen@intel.com>

hypervisor/include/public/acrn_common.h

@@ -833,6 +833,8 @@ struct shared_buf {
 #define VM_EVENT_POWEROFF	1U
 #define VM_EVENT_TRIPLE_FAULT	2U
 
+#define VM_EVENT_COUNT		3U
+
 #define VM_EVENT_DATA_LEN	28U
 
 struct vm_event {