microvm/acpi: use seabios with acpi=on

With acpi=off continue to use qboot.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id: 20200915120909.20838-12-kraxel@redhat.com