[HVM] Re-introduce the 'apic' configuration option, default to 1.

Also simplify the HVM builder interface by doing more work in
the python wrapper, and fix mapping of shared_info page after the
change to map foreign pages by GMFN rather than MFN.

Signed-off-by: Keir Fraser <keir@xensource.com>

tools/python/README.XendConfig

@@ -123,6 +123,7 @@ otherConfig
                                 image.hvm.vncconsole
                                 image.hvm.pae
                                 image.hvm.acpi (also in image.devices)
+                                image.hvm.apic
                                 image.hvm.devices.boot
                                 image.hvm.devices.fda
                                 image.hvm.devices.fdb