libs: Prepare libhyp for move

Trivial changes to limit the diff when moving the files to a minimum:

- use the relative super::hypervisor path in the (private) back-ends;
- get Error and Result directly from crate:: (not crate::error::);
- move extern crate alloc next to cfg(no_std) (can be removed at once);
- remove (private i.e. unused) "use hypervisor::GeniezoneError"

Test: -
Change-Id: I55f5c74d0e6fc78eeb5d066174816a60bb10936c
diff --git a/libs/hyp/src/hypervisor.rs b/libs/hyp/src/hypervisor.rs
index c53b886..dab15ec 100644
--- a/libs/hyp/src/hypervisor.rs
+++ b/libs/hyp/src/hypervisor.rs
@@ -14,14 +14,12 @@
 
 //! Wrappers around hypervisor back-ends.
 
-extern crate alloc;
-
 mod common;
 mod geniezone;
 mod gunyah;
 mod kvm;
 
-use crate::error::{Error, Result};
+use super::{Error, Result};
 use alloc::boxed::Box;
 use common::Hypervisor;
 pub use common::{