Support sparse image to qemu image conversion

Add sparse image magic word check when converting sparse image to qemu
image.

Bug: 63790380
Test: $ mk_qemu_image.sh $OUT/sparse_system.img
      $ mk_qemu_image.sh $OUT/raw_system.img
Test: Built on Mac and do the commands above

Change-Id: Ice0e62b7b51bab55b51dcb2f52c53ac8aebb6b8e
1 file changed