xen: credit2: optimize runq_candidate() a little bit

xen: credit2: optimize runq_candidate() a little bit

By factoring into one (at the top) all the checks
to see whether current is the idle vcpu, and mark
it as unlikely().

In fact, if current is idle, all the logic for
dealing with yielding, context switching rate
limiting and soft-affinity, is just pure overhead,
and we better rush checking the runq and pick some
vcpu up.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>