xen: rtds: only tickle non-already tickled CPUs

xen: rtds: only tickle non-already tickled CPUs

When more than one idle VCPUs that have the same PCPU as their
previous running core invoke runq_tickle(), they will tickle the same
PCPU. The tickled PCPU will only pick at most one VCPU, i.e., the
highest-priority one, to execute. The other VCPUs will not be
scheduled for a period, even when there is an idle core, making these
VCPUs unnecessarily starve for one period.

Therefore, always make sure that we only tickle PCPUs that have not
been tickled already.

Signed-off-by: Haoran Li <naroahlee@gmail.com>
Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>