It's better to add a hint to indicate that the image should be cloud based distro when the "Installation type" is "Cloud base image"

[yunmingyang]

Description of problem:
When using "Cloud base image" for "Installation type", it's better to add a hint to indicate that the image should be cloud based distro(maybe the image must contain cloud-init) so that the configuration passed by the VM creation could take effect. Or it may cause some confused about the configuration passed by the VM creation is not applied if using a image without cloud-init

Version-Release number of selected components (if applicable):
cockpit-300.1-1.el8_9.x86_64
cockpit-machines-301.1.2.gb0140b7-1.fc38.noarch
libvirt-dbus-1.3.0-2.module+el8.9.0+18724+20190c23.x86_64

How reproducible:
100%

Steps to Reproduce:
1 Log in cockpit, then switch the machines page
2 Click "Create VM", then select "Cloud base image" for "Installation type"

Actual results:
1 After step 2, no hint to indicate that the image must contain cloud-int

Expected results:
1 After step 2, there should be a hint to indicate that the image must contain cloud-init

Additional info: