[pvmfw] Move MemorySharer from pvmfw to vmbase

This cl mvoes MemorySharer and two utility functions
virt_to_phys and phys_to_virt from pvmfw to vmbase for reuse in
rialto later.

Bug: 284462758
Test: m pvmfw_img
Change-Id: I48a410792370beaa531ea0408670b8d831150272
diff --git a/vmbase/src/lib.rs b/vmbase/src/lib.rs
index d086f1c..2541b8a 100644
--- a/vmbase/src/lib.rs
+++ b/vmbase/src/lib.rs
@@ -16,6 +16,8 @@
 
 #![no_std]
 
+extern crate alloc;
+
 pub mod arch;
 mod bionic;
 pub mod console;