pvmfw: Refactor pKVM HVCs

Instead of assuming that pKVM is the only hypervisor that pvmfw will
run on, introduce a new module to host the back-end interfaces and
clarify that the HVCs we currently have are pKVM-specific.

Note: no functional change intended.

Bug: 271493784
Test: TH
Change-Id: Ie96556afbdff585a87319046e3b0dc9bf00b87f9
diff --git a/pvmfw/src/main.rs b/pvmfw/src/main.rs
index d89e718..577ad6e 100644
--- a/pvmfw/src/main.rs
+++ b/pvmfw/src/main.rs
@@ -31,6 +31,7 @@
 mod heap;
 mod helpers;
 mod hvc;
+mod hypervisor;
 mod instance;
 mod memory;
 mod mmio_guard;