vmm: fix immediate_exit handling for PIO/MMIO code paths

To finish the emulation of PIO or MMIO code paths [0], we need to get
back into KVM_RUN. In order to not lose any signal that we received,
we must preserve the original `immediate_exit` flag in case it was
already set by the signal handler.

[0] https://elixir.bootlin.com/linux/v6.12/source/arch/x86/kvm/x86.c#L11555

Author: phip1611

vmm/src/cpu.rs

@@ -1165,12 +1165,18 @@ impl CpuManager {
 
                                 #[cfg(feature = "kvm")]
                                 if matches!(hypervisor_type, HypervisorType::Kvm) {
+                                    let old_immediate_exit_flag = {
+                                        let kvm_run: *mut kvm_run = KVM_RUN.get();
+                                        // SAFETY: the mapping is valid
+                                        let kvm_run = unsafe { kvm_run.as_mut().unwrap() };
+                                        kvm_run.immediate_exit != 0
+                                    };
                                     vcpu.lock().as_ref().unwrap().vcpu.set_immediate_exit(true);
                                     if !matches!(vcpu.lock().unwrap().run(), Ok(VmExit::Ignore)) {
                                         error!("Unexpected VM exit on \"immediate_exit\" run");
                                         break;
                                     }
-                                    vcpu.lock().as_ref().unwrap().vcpu.set_immediate_exit(false);
+                                    vcpu.lock().as_ref().unwrap().vcpu.set_immediate_exit(old_immediate_exit_flag);
                                 }
 
                                 vcpu_run_interrupted.store(true, Ordering::SeqCst);