cpus: protect all icount computation with seqlock

cpus: protect all icount computation with seqlock

Move the icount->ns computation to cpu_get_icount, and make
cpu_get_icount_locked return the raw value.  This makes the
atomic_read__nocheck safe, because it now happens always inside a
seqlock and any torn reads will be retried.  qemu_icount_bias and
icount_time_shift also need to be accessed with atomics.  At the
same time, however, you don't need atomic_read within the writer,
because no concurrent writes are possible.

The fix to vmstate lets us keep the struct nicely packed.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>