x86/IO-APIC: fix setting of destinations

x86/IO-APIC: fix setting of destinations

In commit a85da715cf ("x86/IO-APIC: adjust setting of destinations") I
made a pretty blatant mistake: get_apic_id() can be used there only
when running APICs in physical mode. For both flat and clustered modes
the change was wrong, causing different kinds of boot problems on
affected systems. Don't revert that change though, but use TARGET_CPUS
(equaling cpu_online_map, and with there only being a single online CPU
fulfilling the original commits intention).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>