vl: clean up -boot variables

Move more of them into MachineState, in preparation for moving initialization
of the machine out of vl.c.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/hw/boards.h

@@ -283,6 +283,7 @@ struct MachineState {
     ram_addr_t maxram_size;
     uint64_t   ram_slots;
     const char *boot_order;
+    const char *boot_once;
     char *kernel_filename;
     char *kernel_cmdline;
     char *initrd_filename;