PCMachineState: introduce acpi_build_enabled field

Introduce this field to control whether ACPI build is enabled by a
particular machine or accelerator.

It defaults to true if the machine itself supports ACPI build. Xen
accelerator will disable it because Xen is in charge of building ACPI
tables for the guest.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>