vfio: selftests: Store libvfio build outputs in $(OUTPUT)/libvfio

vfio: selftests: Store libvfio build outputs in $(OUTPUT)/libvfio

Store the tools/testing/selftests/vfio/lib outputs (e.g. object files)
in $(OUTPUT)/libvfio rather than in $(OUTPUT)/lib. This is in
preparation for building the VFIO selftests library into the KVM
selftests (see Link below).

Specifically this will avoid name conflicts between
tools/testing/selftests/{vfio,kvm/lib and also avoid leaving behind
empty directories under tools/testing/selftests/kvm after a make clean.

Link: https://lore.kernel.org/kvm/20250912222525.2515416-2-dmatlack@google.com/
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250922224857.2528737-1-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>