libvirt: remove redundant 'libvirt_' prefix in test case names

All the virt driver tests were moved into subdirectories
which are separate for each virt driver. There is thus no
reason to still have 'libvirt_' as a prefix in the filename
too.

A further problem with the current naming is that if you
want to just run the 'test_libvirt' test, a substring match
on that will also pick up other test suites (eg it will match
test_libvirt_config) and run those too. The renaming ensures
that no test name is a valid prefix for another test name,
making it easier to specify exactly which test file you want
run.

Change-Id: I9fe3dee5d7b7b834de84b719cab993969e34d16b
Merge "libvirt: remove redundant 'libvirt_' prefix in test case names"