vl.c/exit: pause cpus before closing block devices

There's a rare exit seg if the guest is accessing
IO during exit.
It's always hitting the atomic_inc(&bs->in_flight) with a NULL
bs. This was added recently in 99723548  but I don't see it
as the cause.

Flip vl.c around so we pause the cpus before closing the block devices,
that way we shouldn't have anything trying to access them when
they're gone.

This was originally Red Hat bz https://bugzilla.redhat.com/show_bug.cgi?id=1451015

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reported-by: Cong Li <coli@redhat.com>

--
This is a very rare race, I'll leave it running in a loop to see if
we hit anything else and to check this really fixes it.

I do worry if there are other cases that can trigger this - e.g.
hot-unplug or ejecting a CD.

Message-Id: <20170713190116.21608-1-dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
