qtest: Fix the bug about disable vnc causes "make check" fail

When we disable vnc from "./configure", QEMU can't use the vnc option.
So qtest can't use the "vnc -none ", otherwise "make check" fails.
If QEMU uses "-display none", "-vnc none" is excrescent, So we just need to drop it.

Signed-off-by: Kewei Yu <keweihk@gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
