Antivirus programs operate at the file level, that is, on every filesystem access (open, read, write, etc) they intercede on the request and filter it according to their various rules. For example, checking the contents of the file for viral signatures if it's an executable.
Virtual Machine hosts operate at the cpu level, that is they intercede on their Guest OS's when the OS does something that is very low level and needs to be emulated to preserve the state of the VM. The Host VM software is oblivious to the actual operation of the Guest OS, and operates simply to continue the illusion that the Guest OS is running on real hardware (this isn't strictly true, various tools exists to allow the Guest OS access to the Host OS, VMWare tools drivers for example).
The only way for this to work would be a driver loaded in the Guest OS that delegated control on each access to the Host VM and from there to a user space AV program running on the host. The result of those operations would have to be propagated back into the Guest OS to be applied by it's OS to avoid corruption from the Guest and the Host modifying the disk structures concurrently.
So, at the end of the day you would have one copy of your AV software running external to the Guest OS, but having to communicate with all of them (in a very complicated manor) to achieve what a traditional AV solution would do.
We haven't even touched on dealing with running multiple heterogeneous OS's on the same Host VM, upgrades to this shared AV service, VM migration between physical hosts, etc.
I doubt that the solution above would prove to be as performant as a traditional AV service running inside the Guest OS. What was your motivation for asking this question ?