Although the canonical source of reference for QMP commands is
qapi-schema.json, for consistency's sake, update qmp-commands.hx to
state the list of supported transactionable operations, namely:

    drive-backup
    blockdev-backup
    blockdev-snapshot-internal-sync
    abort
    block-dirty-bitmap-add
    block-dirty-bitmap-clear

Also update the possible values for the "type" action array.
Signed-off-by: NKashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>