kvm: extract RBD snapshot unprotect into a helper method

[calvix]

Follow-up to the reviewer's "I would personally still give the snapprotect block a separate method as well":

The snapUnprotect block in the finally clause of createRBDvolumeFromRBDSnapshot moves into a never-throwing unprotectRbdSnapshot(image, snapshotName, snapProtected) helper next to closeRbdImage/destroyRadosIoCtx. The snapProtected guard moves inside the helper, mirroring how the other helpers guard on null — the finally clause is now four plain helper calls with no inline try-catch at all:

} finally {
    closeRbdImage(rbd, diskImage, newUuid);
    unprotectRbdSnapshot(srcImage, snapshotName, snapProtected);
    closeRbdImage(rbd, srcImage, volume.getName());
    destroyRadosIoCtx(r, io, snapshotName);
}

Keeps ERROR severity and the operator-facing message about the snapshot/volume being undeletable until resolved.

No behavior change. All 39 tests in KVMStorageProcessorTest pass.