To convert from vmware to libvirt, I'd do:
virt-convert myvm.vmx -o virt-image IMAGE.XML virt-image IMAGE.XML
but how do I create an image descriptor from a libvirt domain to be able to reverse the export back to vmware?
Converting from domain XML config to VMware VMX config. The virsh domxml-to-native provides a way to convert a domain XML config into a VMware VMX config. This is a new tool without much documentation yet, but no options should really be needed. https://libvirt.org/drvesx.html
This was an older solution:
Under Convert Domain XML Format Configuration File to libvirt “image” XML Configuration File (virt-image). http://virtuallyhyper.com/2013/06/migrate-from-libvirt-kvm-to-virtualbox/