I'm sorry, I think you misunderstand the technology. Hardware virtualization covers much of the CPU cases, but there are still three attack surfaces: handling the guest instruction traps on the host side which can actually get quite hairy in corner cases, the devices the guest drives, implemented in software on the host side, and shared hardware information leakage.

These are all improving: hardware support is generally getting better and guests modified to behave nicer so weird cases can just fail, and devices are often now specifically designed for virtualization (plug for my old project VirtIO here!). But it's still software, and asserting there are no bugs left seems incautious.

The information leakage problems are an ongoing whack-a-mole which depends on your setup: they may or may not result in escapes, depending on the nature of the secrets.

Of course, higher level bugs are still there: convincing people or software to change settings, install software or pass too much information through existing channels which are perfectly secure!

Google: qubes escape

-- they're rarely discovered. But they have existed, more likely currently exist or will exist. Certainly way better than not having that protection, but running sketchy binaries is still not the best idea!