block: Accept node-name for block-commit

block: Accept node-name for block-commit

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

As libvirt makes use of the DeviceNotFound error class, we must add
explicit code to retain this behaviour because qmp_get_root_bs() only
returns GenericErrors.

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