block: asynchronously stop the VM on I/O errors

With virtio-blk dataplane, I/O errors might occur while QEMU is
not in the main I/O thread.  However, it's invalid to call vm_stop
when we're neither in a VCPU thread nor in the main I/O thread,
even if we were to take the iothread mutex around it.

To avoid this problem, we can raise a request to the main I/O thread,
similar to what QEMU does when vm_stop is called from a CPU thread.
We know that bdrv_error_action is called from an AIO callback, and
the moment at which the callback will fire is not well-defined; it
depends on the moment at which the disk or OS finishes the operation,
which can happen at any time.  Note that QEMU is certainly not in a CPU
thread and we do not need to call cpu_stop_current() like vm_stop() does.

However, we need to ensure that any action taken by management will
result in correct detection of the error _and_ a running VM.  In particular:

- the event must be raised after the iostatus has been set, so that
"info block" will return an iostatus that matches the event.

- the VM must be stopped after the iostatus has been set, so that
"info block" will return an iostatus that matches the runstate.

The ordering between the STOP and BLOCK_IO_ERROR events is preserved;
BLOCK_IO_ERROR is documented to come first.

This makes bdrv_error_action() thread safe (assuming QMP events are,
which is attacked by a separate series).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>