This makes usable default for -cpu option both for qemu-system-xtensa
and qemu-system-xtensaeb fixing the following error:

    $ qemu-system-xtensaeb -M sim
    Unable to find CPU definition
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Tested-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>