KVM: ia64: dynamic nr online cpus

Account for number of online cpus and use that in loops iterating over
the list of vpus instead of scanning the full array unconditionally.
This patch is a building block to facilitate allowing to bump up
the size of MAX_VCPUS significantly.

Signed-off-by: Jes Sorensen <jes@sgi.com>
Acked-by : Xiantao Zhang  <xiantao.zhang@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>