create: improve interface when attempting to create docker driver

[jedevc]

Fix https://github.com/docker/buildx/issues/1269.

Previously, the help information for buildx indicated that users could create a new instance of the docker driver - which is explicitly something we don't support, driver of this form are automatically derived from the available list of docker contexts.

This patch ensures that don't have AllowsInstance set will not appear in the help text, and additionally provide a new more specific error message instead of the generic "failed to find driver". This should help point users in the correct direction.