virt: Add virt directory to the top Makefile

We need to build files in virt/lib/, which are now used by
KVM and VFIO, so add virt directory to the top Makefile.

Signed-off-by: Feng Wu <feng.wu@intel.com>
Acked-by: Michal Marek <mmarek@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>