rules.mak: speedup save-vars load-vars

Unnesting variables spends a lot of time parsing and executing foreach
and if functions.  Because actually very few variables have to be
saved and restored, a good strategy is to remember what has to be done
in load-vars, and only iterate the right variables in load-vars.
For save-vars, unroll the foreach loop to provide another small
improvement.

This speeds up a "noop" build from around 15.5 seconds on my laptop
to 11.7 (25% roughly).

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
