libvmm: explicitly save and restore errno in vm_open()

In commit 6bb140e3ca, vm_destroy() was replaced with free() to
preserve errno. However, it's possible that free() may change the errno
as well. Keep the free() call, but explicitly save and restore errno.

Noted by: jhb
Fixes: 6bb140e3ca

lib/libvmmapi/vmmapi.c

@@ -118,6 +118,7 @@ struct vmctx *
 vm_open(const char *name)
 {
 	struct vmctx *vm;
+	int saved_errno;
 
 	vm = malloc(sizeof(struct vmctx) + strlen(name) + 1);
 	assert(vm != NULL);
@@ -133,7 +134,9 @@ vm_open(const char *name)
 
 	return (vm);
 err:
+	saved_errno = errno;
 	free(vm);
+	errno = saved_errno;
 	return (NULL);
 }