option: Tweak invalid size error message and unbreak iotest 049

Commit 75cdcd1 neglected to update tests/qemu-iotests/049.out, and
made the error message for negative size worse.  Fix that.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
