Close the device file descriptor after the validation phase (#48)
Close the device file descriptor after the validation phase (#48)

Not doing so causes `EBUSY` when attempting to unmount the drive right
afterwards in some Windows 7 systems.

See: https://github.com/resin-io/etcher/issues/573
Signed-off-by: Juan Cruz Viotti <8493db2a4f45d66df375afe12f7eba59fe1c8d78@gmail.com>
resin-io/resin-image-write,resin-io-modules/resin-image-write,resin-io-modules/etcher-image-write