hypervisor: Implement get_device_attr method for AArch64

This commit implements the `get_device_attr` method for the
`KVM_GET_DEVICE_ATTR` ioctl. This ioctl will be used in retrieving
the GIC status.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>

arch/src/aarch64/gic/mod.rs

@@ -15,6 +15,8 @@ pub enum Error {
     CreateGIC(hypervisor::HypervisorVmError),
     /// Error while setting device attributes for the GIC.
     SetDeviceAttribute(hypervisor::HypervisorDeviceError),
+    /// Error while getting device attributes for the GIC.
+    GetDeviceAttribute(hypervisor::HypervisorDeviceError),
 }
 type Result<T> = result::Result<T, Error>;
 
@@ -111,6 +113,27 @@ pub mod kvm {
             Ok(())
         }
 
+        /// Get a GIC device attribute
+        fn get_device_attribute(
+            device: &Arc<dyn hypervisor::Device>,
+            group: u32,
+            attr: u64,
+            addr: u64,
+            flags: u32,
+        ) -> Result<()> {
+            let mut attr = kvm_bindings::kvm_device_attr {
+                group,
+                attr,
+                addr,
+                flags,
+            };
+            device
+                .get_device_attr(&mut attr)
+                .map_err(super::Error::GetDeviceAttribute)?;
+
+            Ok(())
+        }
+
         /// Finalize the setup of a GIC device
         fn finalize_device(gic_device: &dyn GICDevice) -> Result<()> {
             /* We need to tell the kernel how many irqs to support with this vgic.