With no Virtual Box in Debian Buster @ryecoaaron has implemented Cockpit which can be installed from OMV-Extras the version installed is 202.1 this version brings a fundamental change particularly in relation to this thread.
The version available for OMV4 is 188, if you deploy a Virtual Machine the VM is installed on your boot drive it is not possible to install/deploy on a shared folder on your server. The new version, 202.1 with OMV5 will allow you to do this by creating a Storage Pool from within the Virtual Machines Menu, to enable that you will need to install two additional packages -> libvirt-dbus and cockpit-storaged.
apt-get install libvirt-dbus
apt-get install cockpit-storaged
Have I tested this 
I've tested the install to ensure the additional packages appear in the menu. 