virt-manager 1.5.1-1 (Debian Buster [Testing])
Neither the Connection Details > Network Interfaces window, nor the Virtual Network Interface device configuration in the domain configuration panel in virt-manager allow you to set the virtual link state of a virtual interface, you must use virsh's 'domif-setlink' command to do so.
Libvirt manages interface link states using the <link> element in the domain XML, and the state attribute of that link element.  This is how 'virsh domif-setlink' works. 
This would be a useful feature to include in the UI, so you can pull a VM off of the network easily if you need to without affecting the host, or other VMs, since this directly affects the virtual NIC rather than a bridge. (Without needing to use virsh)
My concept for implementation is to add a "Link state:" label with a toggle button next to it, to the Virtual Network Interface device page in domain info. Activating the button will toggle the link state up or down, similar to domif-setlink.  I'm not very attached to this implementation concept if you have a better one.
I swear, I did do extensive searching of this tracker for any duplicate reports, but alas, I missed bug 1614532. I discovered this after a little bit of research on the github, and finding out my RFE is already complete, just not in Debian Buster yet.
*** This bug has been marked as a duplicate of bug 1614532 ***