vfio-ccw: protect the I/O region

Introduce a mutex to disallow concurrent reads or writes to the
I/O region. This makes sure that the data the kernel or user
space see is always consistent.

The same mutex will be used to protect the async region as well.

Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>