[build] Pad .rom, .dsk, and .hd images to 512-byte boundaries

QEMU will silently round down a disk or ROM image file to the nearest
512 bytes.  Fix by always padding .rom, .dsk and .hd images to the
nearest 512-byte boundary.

Originally-fixed-by: Stefan Hajnoczi <stefanha@gmail.com>

src/Makefile.housekeeping

@@ -724,6 +724,7 @@ define media_template
 	@$(ECHO_E) '$$(BIN)/%.$(1) : $$(BIN)/%.$(1).zbin' \
 		  '\n\t$$(QM)$(ECHO) "  [FINISH] $$@"' \
 		  '\n\t$$(Q)$$(CP) $$< $$@' \
+		  '\n\t$$(Q)$$(PAD_$(1))' \
 		  '\n\t$$(Q)$$(FINALISE_$(1))' \
 		> $(2)