[build] Add %.vhd target for building VM bootable disk images

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

 GENKEYMAP	:= ./util/genkeymap.pl
 DOXYGEN		:= doxygen
 LCAB		:= lcab
+QEMUIMG		:= qemu-img
 
 ###############################################################################
 #

src/arch/x86/Makefile.pcbios

 	$(QM)$(ECHO) "  [FINISH] $@"
 	$(Q)cat $^ > $@
 
+NON_AUTO_MEDIA	+= vhd
+%vhd: %usb
+	$(QM)$(ECHO) "  [FINISH] $@"
+	$(Q)$(QEMUIMG) convert -f raw -O vpc $< $@
+
 # Padded floppy image (e.g. for iLO)
 NON_AUTO_MEDIA += pdsk
 %pdsk : %dsk