x86: restore ability to work on systems without APIC

x86: restore ability to work on systems without APIC

This got broken with the default-enabling of MSI. Apart from fixing
the base issue, the patch also addresses
- the 'i' command crashing where there is no IO-APIC,
- the 'i' command needlessly printing information for all 256 vectors
  when the use of IO-APIC(s) is disabled, and
- the need to specify both "nolapic" and "noapic" when "nolapic" alone
  should already have the intended effect.

Signed-off-by: Jan Beulich <jbeulich@novell.com>