docs: Migration v2 is now no longer draft

docs: Migration v2 is now no longer draft

Add further instructions to the libxc "Future Extensions" section, and
provide such a section for libxl.

In addition, drop the "In experimental __func__" IPRINTF()s from the
libxc implementations.

Finally, a correction to libxl's "Not Yet Included" section which
should have been amended in c/s 7eaec00 when libxl Remus support was
introduced into the protocol.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>