block: Accept node-name for blockdev-backup

block: Accept node-name for blockdev-backup

In order to remove the necessity to use BlockBackend names in the
external API, we want to allow node-names everywhere. This converts
blockdev-backup and the corresponding transaction action to accept a
node-name without lifting the restriction that we're operating at a root
node.

In case of an invalid device name, the command returns the GenericError
error class now instead of DeviceNotFound, because this is what
qmp_get_root_bs() returns.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>