Unless you want to do something really magic[1], you almost certainly
don't want to use kvm to boot a mounted filesystem.  This can cause
filesystem corruption.  Please consider having kvm detect mounted
filesystems and refuse to boot them.  To handle those magic cases, offer
an option -force-mounted or similar to override that detection.

I've done this myself (realized halfway through a Debian install onto a
USB drive via debian-installer in KVM that I left the USB drive booted),
and I've also had to explain how to check for mounted partitions before
booting them with kvm.  A bit of DWIM seems helpful.

[1] Like mounting GFS2 or another multi-mount filesystem, or booting
    your main disk and carefully not booting the running system's root
    filesystem...

- Josh Triplett