tools/libxc: Remove osdep indirection for privcmd

tools/libxc: Remove osdep indirection for privcmd

The alternative backend (a xen-api/xapi shim) is no longer around and
so this stuff is now just baggage which is getting in the way of
refactoring libxenctrl.

Nested virt probably suffices for this use case now.

This was the last component of the osdep infrastructure, so all the
dynamic loading etc stuff all falls away too.

As part of this I was forced to investigate the twisty
xc_map_foreign_* maze, which I have added to the
toolstack-library-apis doc in the hopes of doing something sensible.

NetBSD and Solaris now call xc_map_foreign_bulk_compat directly from
their xc_map_foreign_bulk, which could have been achieved by using
some ifdefs around a renamed function. This will fall out in the wash
when these functions move to their own library.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>