cpus: unify qemu_*_wait_io_event

cpus: unify qemu_*_wait_io_event

Except for round-robin TCG, every other accelerator is using more or
less the same code around qemu_wait_io_event_common.  The exception
is HAX, which also has to eat the dummy APC that is queued by
qemu_cpu_kick_thread.

We can add the SleepEx call to qemu_wait_io_event under "if
(!tcg_enabled())", since that is the condition that is used in
qemu_cpu_kick_thread, and unify the function for KVM, HAX, HVF and
multi-threaded TCG.  Single-threaded TCG code can also be simplified
since it is only used in the round-robin, sleep-if-all-CPUs-idle case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>