vmm: Only return from reset driven I/O once event received

The reset system is asynchronous with an I/O event (PIO or MMIO) for
ACPI/i8042/CMOS triggering a write to the reset_evt event handler. The
VMM thread will pick up this event on the VMM main loop and then trigger
a shutdown in the CpuManager. However since there is some delay between
the CPU threads being marked to be killed (through the
CpuManager::cpus_kill_signalled bool) it is possible for the guest vCPU
that triggered the exit to be re-entered when the vCPU KVM_RUN is called
after the I/O exit is completed.

This is undesirable and in particular the Linux kernel will attempt to
jump to real mode after a CMOS based exit - this is unsupported in
nested KVM on AMD on Azure and will trigger an error in KVM_RUN.

Solve this problem by spinning in the device that has triggered the
reset until the vcpus_kill_signalled boolean has been updated
indicating that the VMM thread has received the event and called
CpuManager::shutdown(). In particular if this bool is set then the vCPU
threads will not re-enter the guest.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

fuzz/fuzz_targets/cmos.rs

@@ -6,6 +6,8 @@
 use devices::legacy::Cmos;
 use libc::EFD_NONBLOCK;
 use libfuzzer_sys::fuzz_target;
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
 use vm_device::BusDevice;
 use vmm_sys_util::eventfd::EventFd;
 
@@ -25,6 +27,7 @@ fuzz_target!(|bytes| {
         u64::from_le_bytes(below_4g),
         u64::from_le_bytes(above_4g),
         EventFd::new(EFD_NONBLOCK).unwrap(),
+        Arc::new(AtomicBool::default()),
     );
 
     let mut i = 16;