Normal it's more save to treat a vm like a normal system, so using an antivirus in the guest os.
However there is also a possiblilty to scan the vmdk file. Its based on VMware, but should be the same for other products.
Symantec Offline Image Scanner (SOIS) is a stand-alone tool that can be >used to scan .vmdk files using Symantec Endpoint Protection (SEP) 12, >Symantec Endpoint Protection (SEP) 11, or Symantec AntiVirus (SAV) 10 virus >definitions.
Short explain about the files:
This is the file that stores the state of the virtual machine's BIOS.
This is a virtual disk file, which stores the contents of the virtual >machine's hard disk drive.
A virtual disk is made up of one or more .vmdk files. If you have specified >that the virtual disk should be split into 2GB chunks, the number of .vmdk >files depends on the size of the virtual disk. As data is added to a >virtual disk, the .vmdk files grow in size, to a maximum of 2GB each. (If >you specify that all space should be allocated when you create the disk, >these files start at the maximum size and do not grow.) Almost all of a >.vmdk file's content is the virtual machine's data, with a small portion >allotted to virtual machine overhead.
If the virtual machine is connected directly to a physical disk, rather >than to a virtual disk, the .vmdk file stores information about the >partitions the virtual machine is allowed to access.
Earlier VMware products used the extension .dsk for virtual disk files.