qemu/tests/cpuid: fix escaping of "-cpu ?" command

The command being run is a shell command, so running it as "-cpu ?"
without any quotes around "?" will make the shell expand "?" to the list
of filenames with one letter in the current directory.
Let's fix this by properly escaping the question mark.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>