The libnbd project has a documented registration system where
third-party implementations can include a link to their stable
documentation of meta-contexts.  Qemu was the first project to utilize
this process[1], by pointing to documentation on its own
meta-contexts[2].  And in libnbd, we even foreshadowed the need to
document future meta-contexts by creating metadata_namespaces as a
list even when it started out with only one element.  It's time to
extend that list to include the qemu contexts.

[1] https://github.com/NetworkBlockDevice/nbd/commit/e96b311caf901
[2] https://gitlab.com/qemu-project/qemu/-/blob/master/docs/interop/nbd.txt

"qemu:dirty-bitmap:NAME" is a set of namespaces for any arbitrary
dirty bitmap name; we can't define a constant for every possible dirty
bitmap name, but it is possible to do NBD_OPT_LIST_META_CONTEXT on
"qemu:dirty-bitmap:" to see which dirty bitmaps are available.  When a
dirty bitmap is negotiated, only one bit is defined (an ...