microvm/acpi: use GSI 16-23 for virtio

With ACPI enabled and IO-APIC being properly declared in the ACPI tables
we can use interrupt lines 16-23 for virtio and avoid shared interrupts.

With acpi disabled we continue to use lines 5-12.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20200915120909.20838-11-kraxel@redhat.com