Adapted from https://github.com/aborche/qemu-guest-agent

Index: qga/main.c
--- qga/main.c.orig
+++ qga/main.c
@@ -44,7 +44,11 @@
 #else /* CONFIG_BSD */
 #define QGA_VIRTIO_PATH_DEFAULT "/dev/virtio-ports/org.qemu.guest_agent.0"
 #endif /* CONFIG_BSD */
+#ifdef __OpenBSD__
+#define QGA_SERIAL_PATH_DEFAULT "/dev/cua01"
+#else
 #define QGA_SERIAL_PATH_DEFAULT "/dev/ttyS0"
+#endif
 #define QGA_STATE_RELATIVE_DIR  "run"
 #else
 #define QGA_VIRTIO_PATH_DEFAULT "\\\\.\\Global\\org.qemu.guest_agent.0"
@@ -1524,7 +1528,11 @@ int main(int argc, char **argv)
     }
 
     if (config->method == NULL) {
+#ifdef CONFIG_BSD
+        config->method = g_strdup("isa-serial");
+#else
         config->method = g_strdup("virtio-serial");
+#endif
     }
 
     socket_activation = check_socket_activation();