x86/apic: Fix typo EIO_ACK -> EOI_ACK and document it

Fix typo in the macro name and document the
reason it has this value. Update users.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: gleb@redhat.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/37867b31b9330690af2e60a2a7c4cb4b1b070caf.1337184153.git.mst@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>