For topologies before 1.4 manage qemu missing

If qemu is missing for topologies before 1.4 we try
to search it in the path.

Ref #921

gns3server/modules/qemu/qemu_vm.py

@@ -80,10 +80,12 @@ class QemuVM(BaseVM):
             try:
                 self.qemu_path = qemu_path
             except QemuError as e:
+                # If the binary is not found for topologies 1.4 and later
+                # search via the platform otherwise use the binary name
                 if platform:
                     self.platform = platform
                 else:
-                    raise e
+                    self.qemu_path = os.path.basename(qemu_path)
         else:
             self.platform = platform