qemu: block: Create helper for creating data for legacy snapshots

qemu: block: Create helper for creating data for legacy snapshots

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: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>