This only works if you never use the host (hypervisor) for anything but launching virtual machines.
If the host is compromised: so is the virtual machine. Keeping it encrypted is about the same security as keeping your wallet encrypted. If you never spend funds, an attacker can't either (assuming the passphrase is secure).
that's where you are wrong buddy... if the encrypted volume is unmounted even with ALL your passwords an attacker would have to somehow scrape the entire volume off your PC.... and given that its 80GB or more in size this could take DAYS or even weeks (with most peoples lousy internet) and if your PC was compromised you could turn it off and open the encrypted volume from a clean PC on different network and move the money before they got to it.
also being inside a virtual machine makes it exponentially more difficult for any malicious trojan to get at your wallet files which would also be encrypted (even if the host is compromised). so for the very brief period that you actually have the encrypted volume OPEN on your PC any attacker would have to scrape your entire VM then OPEN it with your passwords then take the wallets out and OPEN them again with another set of passwords and then spend the money before you noticed.
this is so much more complicated than simply copying your .dat files and logging your keystrokes.
yes any PC that is connected to the internet can still be compromised but if you make the difficulty too hard.. most thieves give up and move on to the next target.