Move HVC constants and wrappers to new module.

Bug: 261439403
Test: ran pVM firmware manually
Change-Id: I5ebf9013efa3a723dd975d36fd4fa0859c154127
diff --git a/pvmfw/src/main.rs b/pvmfw/src/main.rs
index 79b6f57..2f1b42a 100644
--- a/pvmfw/src/main.rs
+++ b/pvmfw/src/main.rs
@@ -26,6 +26,7 @@
 mod fdt;
 mod heap;
 mod helpers;
+mod hvc;
 mod memory;
 mod mmio_guard;
 mod mmu;