libxl: use b_info->{acpi,acpi} when available

libxl: use b_info->{acpi,acpi} when available

b_info->u.hvm.{acpi,apic} are deprecated. But also, on recent libxl
version (4.14) the old one seems to be broken. While libxl part should
be fixed too, update the usage here and at some point drop support for
the old version.
b_info->acpi was added in Xen 4.8
b_info->apic was added in Xen 4.10
Xen 4.10 is the oldest version that still has security support (until
December 2020).

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>