virtio: console: add locking around c_ovq operations

When multiple ovq operations are being performed (lots of open/close
operations on virtio_console fds), the __send_control_msg() function can
get confused without locking.

A simple recipe to cause badness is:
* create a QEMU VM with two virtio-serial ports
* in the guest, do
  while true;do echo abc >/dev/vport0p1;done
  while true;do echo edf >/dev/vport0p2;done

In one run, this caused a panic in __send_control_msg().  In another, I
got

   virtio_console virtio0: control-o:id 0 is not a head!

This also results repeated messages similar to these on the host:

  qemu-kvm: virtio-serial-bus: Unexpected port id 478762112 for device virtio-serial-bus.0
  qemu-kvm: virtio-serial-bus: Unexpected port id 478762368 for device virtio-serial-bus.0

Reported-by: FuXiangChun <xfu@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Reviewed-by: Asias He <asias@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org