Add APIs to yield and get the ID of the VM.

Change-Id: I2e6340d200cfddc7ca9877e91d785c438620b6c6
diff --git a/src/api.c b/src/api.c
index 6be8702..7e6e4e6 100644
--- a/src/api.c
+++ b/src/api.c
@@ -101,6 +101,14 @@
 }
 
 /**
+ * Returns the ID of the VM.
+ */
+int64_t api_vm_get_id(const struct vcpu *current)
+{
+	return current->vm->id;
+}
+
+/**
  * Returns the number of VMs configured to run.
  */
 int64_t api_vm_get_count(void)