VMID 0 will be used by spiceterm for task that run on the host.

- remove command from index (so that pvesh does not list it)
- use POST instead of GET
- correctly check permissions
- return value with status codes and data (allow expections)

The client can generate a data url with that content easily.

And create a data URL to open the external viewer (html 'src'
tag only works with 'GET')

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>

It is not possible to boot from such disks at the moment.