vl.c: make find_default_machine() local

Function find_default_machine() is introduced by commit 2c8cffa599
"vl: make find_default_machine externally visible", and it was used
outside of vl.c until commit a904410af5 "pc_sysfw: remove the rom_only
property".

Commit a904410af5 "pc_sysfw: remove the rom_only property" removed the
only user of find_default_machine() outside vl.c, but neglected to make
it static. Do that now.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Message-Id: <20190405064121.23662-2-richardw.yang@linux.intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

include/hw/boards.h

@@ -57,7 +57,6 @@ void memory_region_allocate_system_memory(MemoryRegion *mr, Object *owner,
 #define MACHINE_CLASS(klass) \
     OBJECT_CLASS_CHECK(MachineClass, (klass), TYPE_MACHINE)
 
-MachineClass *find_default_machine(void);
 extern MachineState *current_machine;
 
 void machine_run_board_init(MachineState *machine);

vl.c

@@ -1488,7 +1488,7 @@ static MachineClass *find_machine(const char *name)
     return mc;
 }
 
-MachineClass *find_default_machine(void)
+static MachineClass *find_default_machine(void)
 {
     GSList *el, *machines = object_class_get_list(TYPE_MACHINE, false);
     MachineClass *mc = NULL;