Whenever the slice of a domU is set to 0, sedf_adjdom() sets extraweight

Whenever the slice of a domU is set to 0, sedf_adjdom() sets extraweight
to 0.  Later, in desched_extra_dom(), if the extrawight is not set, the
vcpu's score is calculated with this:

 /*domain was running in L1 extraq => score is inverse of
   utilization and is used somewhat incremental!*/
   if ( !inf->extraweight )
       /*NB: use fixed point arithmetic with 10 bits*/
       inf->score[EXTRA_UTIL_Q] = (inf->period << 10) /
           inf->slice;

Which can result in a divide by zero.

This changeset adds a comments and additional sanity check to prevent this
case from crashing Xen.

Signed-off-by: Ryan Harper <ryanh@us.ibm.com>