qemu: fix crash when removing <filterref> from interface with update-device

qemu: fix crash when removing <filterref> from interface with update-device

If a domain network interface that contains a <filterref> is modified
"live" using "virsh update-device --live", libvirtd would crash. This
was because the code supporting live update of an interface's
filterref was assuming that a filterref might be added or modified,
but didn't account for removing the filterref, resulting in a null
dereference of the filter name.

Introduced with commit 258fb278, which was first in libvirt v1.0.1.

This addresses https://bugzilla.redhat.com/show_bug.cgi?id=1093301