i386: Expose IOAPIC register definitions even if CONFIG_X86_IO_APIC is not set

KVM reuses the IOAPIC register definitions, and needs them even if the
host is not compiled with IOAPIC support.  Move the #ifdef below so that only
the IOAPIC variables and functions are protected, and the register definitions
are available to all.

Signed-off-by: Avi Kivity <avi@qumranet.com>