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" <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Anthony Perard <[email protected]>
Cc: Artyom Tarasenko <[email protected]>
Cc: David Gibson <[email protected]>
Cc: Eduardo Habkost <[email protected]>
Cc: Gerd Hoffmann <[email protected]>
Cc: Igor Mammedov <[email protected]>
Cc: Mark Cave-Ayland <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Peter Maydell <[email protected]>
Cc: Stefano Stabellini <[email protected]>
Signed-off-by: Laszlo Ersek <[email protected]>
Acked-by: Gabriel Somlo <[email protected]>
Tested-by: Gabriel Somlo <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>