Remove bogus 'unplug' calls from libvirt VIF test

Since the 'get_config' method was introduced to the libvirt
VIF class, separate from the 'plug' method, there is no need
to call the 'unplug' method in the test suite

Blueprint: libvirt-vif-driver
Change-Id: I1c3d055e258117c2657cd72386472dff2e0e08b8
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Make URL length limit configurable

We have an application where user data is sometimes (much) bigger
than 8192 bytes.  EC2 uses 16k by default, I think.