qapi: Allow introspecting fix for savevm's cooperation with blockdev

qapi: Allow introspecting fix for savevm's cooperation with blockdev

'savevm' was buggy as it considered all monitor-owned block device
nodes for snapshot. With the introduction of -blockdev, the common
usage made all nodes including protocol and backing file nodes be
monitor-owned and thus considered for snapshot.

This is a problem since the 'file' protocol nodes can't have internal
snapshots and it does not make sense to take snapshot of nodes
representing backing files.

This was fixed by commit 05f4aced658a02b02. Clients need to be able to
detect whether this fix is present.

Since savevm does not have an QMP alternative, add the feature for the
'human-monitor-command' backdoor which is used to call this command in
modern use.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191018081454.21369-6-armbru@redhat.com>