qemu: monitor: Store internal snapshot names from 'query-named-block-nodes'

qemu: monitor: Store internal snapshot names from 'query-named-block-nodes'

Store the names of internal snapshots present in supported images in the
data we dump from 'query-named-block-nodes' so that the upcoming changes
to the internal snapshot code can access it.

To test this we use the bitmap detection test cases which can be easily
extended to dump this data.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>