resource/ovirt_disk_attachment: Correct checking if a disk attachment has been destroyed

[imjoey]

Signed-off-by: imjoey [email protected]

Fixes #161

Changes proposed in this pull request:

• Parse the rs.Primary.ID into respective ID of vm and disk-attachment and then check the existence of the specific disk-attachment
• Change the parameters of function getVMIDAndDiskID() to better fit for general purpose

Output from acceptance testing:

$ make testacc TEST=./ovirt TESTARGS='-run=TestAccOvirtDiskAttachment_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ovirt -v -run=TestAccOvirtDiskAttachment_basic -timeout 180m
=== RUN   TestAccOvirtDiskAttachment_basic
--- PASS: TestAccOvirtDiskAttachment_basic (2.66s)
PASS
ok      github.com/ovirt/terraform-provider-ovirt/ovirt 2.695s