fw-cfg: bump "x-file-slots" to 0x20 for 2.9+ machine types

fw-cfg: bump "x-file-slots" to 0x20 for 2.9+ machine types

More precisely, the "x-file-slots" count is bumped for all machine types
that:
(a) use fw_cfg, and
(b) are not versioned (hence migration is not expected to work for them
    across QEMU releases anyway), or have version 2.9.

This affects machine types implemented in the following source files:

- "hw/arm/virt.c". The "virt-*" machine type is versioned, and the <= 2.8
  versions already depend on HW_COMPAT_2_8 (see commit e353aac51b944).
  Therefore adding the "x-file-slots" compat values to HW_COMPAT_2_8
  suffices.

- "hw/i386/pc.c". The "pc-i440fx-*" (including "pc-*") and "pc-q35-*"
  machine types are versioned. Modifying HW_COMPAT_2_8 is sufficient here
  too (see commit "pc: Add 2.9 machine-types"). The "isapc" machtype is
  not versioned. The "xenfv" machine type, which uses fw_cfg for direct
  kernel booting, is also not versioned.

- "hw/ppc/mac_newworld.c". The "mac99" machine type is not versioned.

- "hw/ppc/mac_oldworld.c". The "g3beige" machine type is not versioned.

- "hw/sparc/sun4m.c". None of the 9 machine types defined in this file
  appear versioned.

- "hw/sparc64/sun4u.c". None of the 3 machine types defined in this file
  appear versioned.

Cc: "Gabriel L. Somlo" <somlo@cmu.edu>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Artyom Tarasenko <atar4qemu@gmail.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>