Clean libhw subdirs as well

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

Makefile.hw

@@ -18,7 +18,7 @@ all: $(hw-obj-y)
 	@true
 
 clean:
-	rm -f *.o *.d *.a *~
+	rm -f *.o */*.o *.d */*.d *.a */*.a *~ */*~
 
 # Include automatically generated dependency files
 -include $(wildcard *.d */*.d)