Previously, libvirt just appended 'script=' onto the QEMU cmd line
according to what <script path=''/> contained, letting QEMU execute the
script. That was flawed from security POV (you don't want QEMU to be
allowed to execute anything), so newer libvirt (as of [1]) executes the
script now. But the libvirt code doesn't allow this corner case (of
allowing and ignoring an empty script path) whereas apparently the QEMU
code does.
So the Nova setting of '' used to work by accident, but now does not.
[1]
http://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=9c17d66 (autocreate
tap device for ethernet network type)
Closes-Bug: #1649527
Change-Id: I4f97c05e2dec610af22a5150dd27696e1d767896
847952927c