Description of problem:

Cockpit UI only allows users to select Filesystem (path to the 
image on host) or URL source methods.

We should add the option to allow users to select an existing disk image 
while creating a VM.

Version-Release number of selected component (if applicable):

cockpit-machines-184.1-1.el8.noarch

How reproducible:

100%

Steps to Reproduce:
1. Start the creation of a new VM through cockpit.
2. Click the drop down `installation source type`
3. Only filesystem and url are displayed


Actual results:

There's no option to allow users to create a VM with an existing image disk

Expected results:

Cockpit UI should allow users to select existing disk images to boot/create 
a VM.