Hi,

it's not easy to figure out which image name is available and which ones are working when creating a new machine

when using ubuntu-xenial:

Error creating machine: Error in driver during machine creation: Too many candidates for ubuntu-xenial (2)

when using ubuntu-bionic there's an error because sudo is not found

but it worked with debian-stretched

I think the issue with ubuntu-xenial should be fixed and maybe a documentation about the available images be written (with indication on where to report bugs)