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/lib.rs b/libs/hyp/src/lib.rs
index 6a23585..2e19bbc 100644
--- a/libs/hyp/src/lib.rs
+++ b/libs/hyp/src/lib.rs
@@ -16,14 +16,14 @@
 
 #![no_std]
 
+extern crate alloc;
+
 mod error;
 mod hypervisor;
 mod util;
 
-pub use crate::hypervisor::DeviceAssigningHypervisor;
 pub use error::{Error, Result};
 pub use hypervisor::{
-    get_device_assigner, get_mem_sharer, get_mmio_guard, KvmError, MMIO_GUARD_GRANULE_SIZE,
+    get_device_assigner, get_mem_sharer, get_mmio_guard, DeviceAssigningHypervisor, KvmError,
+    MMIO_GUARD_GRANULE_SIZE,
 };
-
-use hypervisor::GeniezoneError;