support/qemu-guest: Remove SGA bios parameter and warning

support/qemu-guest: Remove SGA bios parameter and warning

Starting with QEMU version 8.0, the `-device sga` parameter is removed from
the command line because it is no longer needed. These versions include a
SeaBIOS version that contains native support for serial consoles. If an
older version of QEMU is used with `qemu-guest`, the BIOS output may no
longer be visible but the guest would still be able to boot.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Alexander Jung <alex@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #1034