[docs]: qemu tutorial doesn't define vendor-data metadata

# Documentation request
In the tutorial, [user-data](https://cloudinit.readthedocs.io/en/latest/tutorial/qemu.html#what-is-user-data) is defined, but [metadata](https://cloudinit.readthedocs.io/en/latest/tutorial/qemu.html#define-our-metadata) and [vendor-data](https://cloudinit.readthedocs.io/en/latest/tutorial/qemu.html#define-our-vendor-data) are not. We should really include a short description.

The text might say something like:

### What is vendor-data?

Clouds sometimes require a way to provide a minimal configuration.
Vendor-data is cloud-specific, and allows things like providing user-provided ssh key to the instance, and networking configuration for dynamic cloud environments.

Some clouds also provide network configuration to help the instance handle a dynamic network environment. This isn't typically something that a cloud-init user modifies, unless they are managing their own "cloud" environment.

### What is metadata?

This is required, and defines the instance-id. Cloud images are often built from images which have already booted (and therefore previously ran cloud-init). Some method of identifying whether to boot cloud-init as if it is the first time is required. The instance-id is that unique identifier. If instance-id changes, cloud-init will re-run as if it is the first time.