qemu-iotests: Fail test if explicit test case number is unknown

qemu-iotests: Fail test if explicit test case number is unknown

When we expand a number range, we just print "$id - unknown test,
ignored", this is convenient if we want to run a range of tests.

When we designate a test case number explicitly, we shouldn't just
ignore it if the case script doesn't exist.

Print an error and fail the test.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>