hypervisor: Add VmExit::Debug for x86/KVM

This commit adds `VmExit::Debug` for x86/KVM. When the guest hits a
hardware breakpoint, `VcpuExit::Debug` vm exit occurs. This vm exit
will be handled with code implemented in the following commits.

Signed-off-by: Akira Moroo <retrage01@gmail.com>

hypervisor/src/cpu.rs

@@ -266,6 +266,8 @@ pub enum VmExit<'a> {
     Hyperv,
     #[cfg(feature = "tdx")]
     Tdx,
+    #[cfg(feature = "kvm")]
+    Debug,
 }
 
 ///