libxl: add named enum for timer mode.

libxl: add named enum for timer mode.

Unlike previous iterations of this patch the enum values now match the
underlying domctl values.

I looked at updating xl.cfg(5) for these while I was here but frankly, even
after reading the comment in xen/include/public/hvm/params.h, I don't have a
clue what they mean, no_missed_ticks_pending in particular might as well be
written in klingon...

For the same reason I didn't try and give the enum more user-friendly names.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>