tools/libxc: Remove osdep indirection for xc_evtchn

tools/libxc: Remove osdep indirection for xc_evtchn

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.

Note that the intention is to move this into a separate library
shortly.

Nested virt probably suffices for this use case now.

One incorrect instance of using xc_interface where xc_evtchn (in ocaml
stubs) is removed, this used to work because they were typedefs to the
same struct, but is no longer permitted.

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