We need this for all tests that use virHostdevManager, because
during creation of this object for unprivileged connections
like those used in the test suite we would end up writing inside
the user's home directory.

That's bad manners in general, but when running the test suite
inside a purposefully constrained environment such as the one
exposed by pbuilder, it turns into an outright test failure:

  Could not initialize HostdevManager - operation failed: Failed
  to create state dir '/nonexistent/.cache/libvirt/hostdevmgr'
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>