virsh_attach_detach_disk: attach tmp disk with persistent option

Instead of using --config option we will use --persistent option because this
will ensure that the disk will be attached correctly into dead or live guest.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
virsh_attach_detach_disk: attach second temp disk for the detach_twice test

We have to attach two disks for the detach_twice test in order to run the test.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
virsh_attach_detach_disk: make the proper return status checking

It's obvious that if you are running the attach/detach config test that you will
not see the change in the xml returned by dumpxml of a live guest.

This patch also fixes whitespace issues and typo in error message.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>