virsh_attach_detach_disk: do not destroy and start the guest more than once during preparation

It's pointless to destroy and start the guest more than once. It also fixes the issue
that attaching the temporary disk for detach test fails for not IDE disk because we were
trying to attach disk with --config option into 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>