pc: acpi-build: generate pvpanic device description dynamically

Drops AML template patching and allows to
save some space in SSDT if pvpanic device doesn't
exist by not including disabled device description
into SSDT. It also makes device description
smaller by replacing _STA method with named value
and dropping _INI method.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>