Here is the solution that I've found for now. With the name of a virtual machine's domain, it returns the absolute path of the volume used by the domain.
import libvirt
from xml.etree import ElementTree as ET
URI = "qemu:///system"
VM = "truffles"
# Get the virDomain object
conn = libvirt.open(URI)
domain_object = conn.lookupByName(VM)
# Get the XML description of the VM
vm_xml = domain_object.XMLDesc(0)
# Get the volume in use from the element tree
root = ET.fromstring(vm_xml)
disk_source = root.find('./devices/disk/source')
volume_in_use = disk_source.get('file')
print volume_in_use