libvirt.tests.svirt_install: Correct unattended_install import

Now key tests, such as unattended_install were moved to
the virttest.tests namespace. Fix the import link.

CC: Feng Yang <fyang@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #6 from autotest/fix-svirt-install

libvirt.tests.svirt_install: Correct unattended_install import