vl: Report accelerator not supported for target more nicely

When you ask for an accelerator not supported for your target, you get
a bogus "accelerator does not exist" message:

  $ qemu-system-arm -machine none,accel=kvm
  KVM not supported for this target
  "kvm" accelerator does not exist.
  No accelerator found!

Suppress it.
Signed-off-by: NChen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

vl.c

@@ -2740,7 +2740,7 @@ static int configure_accelerator(QEMUMachine *machine)
                 if (!accel_list[i].available()) {
                     printf("%s not supported for this target\n",
                            accel_list[i].name);
-                    continue;
+                    break;
                 }
                 *(accel_list[i].allowed) = true;
                 ret = accel_list[i].init(machine);