cpus: take seqlock across qemu_icount updates

cpus: take seqlock across qemu_icount updates

Even though writes of qemu_icount can safely race with reads in
qemu_icount_raw, qemu_icount is also read by icount_adjust, which
runs in the I/O thread.  Therefore, writes do needs protection of
the vm_clock_lock; for simplicity the patch protects it with both
seqlock+spinlock, which we already do for hosts that lack 64-bit atomics.

The bug actually predated the introduction of vm_clock_lock;
cpu_update_icount would have needed the BQL before the spinlock was
introduced.

Reported-by: Emilio G. Cota <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>