With 'transaction' support we don't need to keep around the multipurpose
code which would create the snapshot if 'transaction' is not supported.

To simplify this add a new helper that just wraps the arguments for
'blockdev-snapshot-sync' operation in 'transaction' and use it instead
of qemuBlockSnapshotAddLegacy.

Additionally this allows to format the arguments prior to creating the
file for simpler cleanup.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>