xen: rtds: Update last_start whenever cur_budget is updated

xen: rtds: Update last_start whenever cur_budget is updated

Make budget accounting code more consistent by making sure the values
used to compute how much budget has been consumed are updated together.

This makes code resilient to calling burn_budget() from more than just
one place -- in case we will need to do that -- without risking subtle
bugs.

No functional changes are intended.

Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>
Acked-by: Dario Faggioli <dario.faggioli@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>