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/mmio_guard.rs b/pvmfw/src/mmio_guard.rs
index e5f376e..dac26e0 100644
--- a/pvmfw/src/mmio_guard.rs
+++ b/pvmfw/src/mmio_guard.rs
@@ -15,7 +15,7 @@
 //! Safe MMIO_GUARD support.
 
 use crate::helpers;
-use crate::hvc::{mmio_guard_enroll, mmio_guard_info, mmio_guard_map, mmio_guard_unmap};
+use crate::hypervisor::{mmio_guard_enroll, mmio_guard_info, mmio_guard_map, mmio_guard_unmap};
 use crate::smccc;
 use core::{fmt, result};