According to xm.config manual, HVM pae|apic|acpi feature default
is 1 (enabled). But in conversion from xm config to libvirt xml,
if xm config doesn't contain pae|apic|acpi, it sets default value
to 0, this causes some problems in HVM guest.
Update parser codes to set HVM pae|apic|acpi default value to 1
to match xm config convension.