Rename manifest.num_vms to vm_count

Rename variable to conform to Hafnium's style guide.

Change-Id: I0b983c98eb28195bc23ba4947857d0d63a990ae3
diff --git a/src/manifest.c b/src/manifest.c
index fb2b695..dc6ed5b 100644
--- a/src/manifest.c
+++ b/src/manifest.c
@@ -268,7 +268,7 @@
 			found_primary_vm = true;
 		}
 
-		manifest->num_vms = i + 1;
+		manifest->vm_count = i + 1;
 		TRY(parse_vm(&vm_node, &manifest->vm[i], vm_id));
 	}