vl.c: make find_default_machine() local

Function find_default_machine() is introduced by commit 2c8cffa5
"vl: make find_default_machine externally visible", and it was used
outside of vl.c until commit a904410a "pc_sysfw: remove the rom_only
property".

Commit a904410a "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>