x86/xen/apic: Mark apic __ro_after_init

Nothing can change it post init.

Remove the 32bit callbacks and comments as XENPV is strictly 64bit.

While at it mop up the whitespace damage which causes eyebleed due to an
editor which is highlighting it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)
Cc: Juergen Gross <jgross@suse.com>