incusd/instance/qemu: Start using seabios as CSM firmware

This adds support for directly using seabios as the CSM firmware rather
than using EDK2's chain boot feature. It also makes this behavior be the
preferred one for new CSM-enabled VMs.

The reason for this change is the removal of CSM support from recent EDK2.

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

internal/server/instance/drivers/driver_qemu.go

@@ -132,6 +132,7 @@ var ovmfSecurebootFirmwares = []ovmfFirmware{
 }
 
 var ovmfCSMFirmwares = []ovmfFirmware{
+	{code: "seabios.bin", vars: "seabios.bin"},
 	{code: "OVMF_CODE.4MB.CSM.fd", vars: "OVMF_VARS.4MB.CSM.fd"},
 	{code: "OVMF_CODE.2MB.CSM.fd", vars: "OVMF_VARS.2MB.CSM.fd"},
 	{code: "OVMF_CODE.CSM.fd", vars: "OVMF_VARS.CSM.fd"},