vdi: make it thread-safe

The VirtualBox driver is using a mutex to order all allocating writes,
but it is not protecting accesses to the bitmap because they implicitly
happen under the AioContext mutex.  Change this to use a CoRwlock
explicitly.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170629132749.997-4-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
