virsh_change_media: Add SKIP option for Enforcing selinux

If the selinux "Enforcing" mode is set, libvirt won't be able to update
the floppy device, so rather than FAIL on a known limitation, set a flag
to just indicate SKIP (along with the reason).

From code review - change /dev/crdom references to /dev/sr0
Merge pull request #1250 from jferlan/virsh_change_media

virsh_change_media: Add SKIP option for Enforcing selinux