Pass x0-x3 boot values to Rust main function.

Bug: 223166344
Test: Ran --unprotected-vm-with-firmware with patched crosvm.
Test: Ran unprotected VM with crosvm.
Change-Id: I1b7d795ab1964741dac1983d4503aa96de818964
diff --git a/vmbase/example/src/main.rs b/vmbase/example/src/main.rs
index e3b9399..3b1786c 100644
--- a/vmbase/example/src/main.rs
+++ b/vmbase/example/src/main.rs
@@ -38,8 +38,9 @@
 main!(main);
 
 /// Entry point for VM bootloader.
-pub fn main() {
+pub fn main(arg0: u64, arg1: u64, arg2: u64, arg3: u64) {
     println!("Hello world");
+    println!("x0={:#010x}, x1={:#010x}, x2={:#010x}, x3={:#010x}", arg0, arg1, arg2, arg3);
     print_addresses();
     check_data();