pull-nbd-2019-01-14

nbd patches for 2019-01-14

Promote bitmap/NBD interfaces to stable for use in incremental
backups. Add 'qemu-nbd --bitmap'.

- John Snow: 0/11 bitmaps: remove x- prefix from QMP api
- Philippe Mathieu-Daudé: qemu-nbd: Rename 'exp' variable clashing with math::exp() symbol
- Eric Blake: 0/8 Promote x-nbd-server-add-bitmap to stable
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