I just created a new test CentOS 7 instance on srv1, in order to work on some quirks with the installer on CentOS 7, and for giggles I decided to let Cloudmin do a Virtualmin install just to make sure it didn't have any quirks of its own...
It gives this error:
Starting installation process ..
.. failed : Virtualmin Pro can only be installed on systems using RPM, Debian or Solaris packages
Which doesn't make sense, as CentOS 7 is clearly a system using RPM. It does however use dnf rather than yum, though there's a wrapper script that'll allow it to work with the same yum commands.
Seems like something is going wrong there, though I can't readily discern what.