qemu-img: refactor dump_map_entry JSON format output

Previously dump_map_entry identified whether we need to start a new JSON
array based on whether start address == 0. In this refactor we remove
this assumption as in following patches we will allow map to start from
an arbitrary position.

Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: Eyal Moscovici <eyal.moscovici@oracle.com>
Message-Id: <20200513133629.18508-4-eyal.moscovici@oracle.com>
Signed-off-by: Eric Blake <eblake@redhat.com>