Ignore offline CPUs when registering IOSAPIC interrupts.  We previously
directed interrupts even to offline CPUs, which means that if you have some
unused CPUs (e.g., you used "maxcpus="), some interrupts won't work.

This really hurts because distro installs typically use "maxcpus=1".

Patch by Alex Williamson.
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>