mini-os: simplify monotonic_clock()

mini-os: simplify monotonic_clock()

monotonic_clock() in arch/x86/time.c is more complex than needed: it
has basically two nested loops making sure the time data obtained from
Xen are valid.

Simplify that by merging some of the used sub-functions into the main
function and using only a single loop. Further simplify the code by
using struct vcpu_time_info for the local instance instead of defining
a similar structure in the code.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>