virt_arm: acpi: reuse common build_fadt()

virt_arm: acpi: reuse common build_fadt()

Extend generic build_fadt() to support rev5.1 FADT
and reuse it for 'virt' board, it would allow to
phase out usage of AcpiFadtDescriptorRev5_1 and
later ACPI_FADT_COMMON_DEF.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>