Unfortunately, this conversion required an additional change.

In the old QMP command, the 'snapshot-file' argument is specified as
optional. The idea is to take the snapshot internally if 'snapshot-file'
is not passed. However, internal snapshots are not supported yet so
the command returns a MissingParamater error if 'snapshot-file' is not
passed. Which makes the argument actually required and will cause
compatibility breakage if we change that in the future.

To fix this the QAPI converted blockdev_snapshot_sync command makes the
'snapshot-file' argument required. Again, in practice it's actually required,
so this is not incompatible.

If we do implement internal snapshots someday, we'll need a new argument
for it.

Note that this discussion doesn't affect HMP.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>