libxl: support mapping files rather than carrying paths around

libxl: support mapping files rather than carrying paths around

This will allow us to map and then unlink the file and therefore
delete the process on process exit or explicit unmap.

Using the mmaped versions of these files required rewriting build_pv
to use the xc_dom builder functionality directly rather than through
the xc_linux_build "compatibility layer". (The status of the
xc_linux_build interface as a compatibility layer seems a bit dubious
since all existing callers use it but if anything is going to replace
it then libxl seems like the likely candidate).

I'm not thrilled with the definition of the maps lifecycle. This could
be solved by adding a helper function to explicitly free the toplevel
structure.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>