Provide warning when switching between distributions

Bootc is available on RHEL, CentOS and Fedora, and I recently accidentally `bootc switch`ed a server from Fedora to CentOS resulting in a broken system (emergency mode in AWS so couldn't change in the grub menu [1]), it would be good if somehow Bootc could detect when you're switching from one distribution to another and provide an appropriate warning.

This was completely my fault, and this operation isn't something I would expect bootc to work with, however a warning or error would help prevent others from making this same mistake.

[1] It's highly likely you can switch to the older grub boot entry from emergency mode, but it's not obvious how to do so (not Bootc's problem though), and the serial console is only available on the more expensive instance types