libhyp: Expose MmioGuardedHypervisor::granule()

Split MmioGuardedHypervisor::init() into the calls it wraps to allow
client code to get the MMIO_GUARD granule from the API.

No functional change intended.

Test: m pvmfw_bin rialto_bin vmbase_example_bin
Change-Id: I8ec81d0bbe372dd06e8ea246954a54cbe2e1af44
diff --git a/vmbase/src/entry.rs b/vmbase/src/entry.rs
index 24b5035..2ff66cc 100644
--- a/vmbase/src/entry.rs
+++ b/vmbase/src/entry.rs
@@ -26,7 +26,8 @@
     console::init();
 
     if let Some(mmio_guard) = get_mmio_guard() {
-        mmio_guard.init()?;
+        mmio_guard.enroll()?;
+        mmio_guard.validate_granule()?;
         mmio_guard.map(console::BASE_ADDRESS)?;
     }