machine: use QAPI struct for boot configuration

As part of converting -boot to a property with a QAPI type, define
the struct and use it throughout QEMU to access boot configuration.
machine_boot_parse takes care of doing the QemuOpts->QAPI conversion by
hand, for now.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414165300.555321-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hw/ppc/prep.c b/hw/ppc/prep.c
index bf622aa..a1cd450 100644
--- a/hw/ppc/prep.c
+++ b/hw/ppc/prep.c
@@ -381,7 +381,7 @@
         }
         boot_device = 'm';
     } else {
-        boot_device = machine->boot_order[0];
+        boot_device = machine->boot_config.order[0];
     }
 
     fw_cfg_add_i16(fw_cfg, FW_CFG_MAX_CPUS, (uint16_t)machine->smp.max_cpus);