Add .vmdk Support

[dorpvom]

By chance I came across the fact (no pun intended) that 7z can extract vmdk files (virtual machine storage files). fact_extractor currently defaults to unblob for these which (potentially) produces a lot of garbage.
This can easily be addressed by adding a vmdk mime type to the 7z plugin. Sadly, there is no mime identification for vmdk, only a full file type (e.g. VMware4 disk image) is given.

In the file repository https://github.com/file/file/magic/Magdir/virtual the two lines corresponding to this are:

0	string/b	VMDK		 VMware4 disk image
0	string/b	KDMV		 VMware4 disk image