sandbox: host bind must close file descriptor

sandbox: host bind must close file descriptor

Each invocation of the 'host bind' command with a file name argument opens
a file descriptor. The next invocation of the 'host bind' command destroys
the block device but the file descriptor remains open. The same holds true
for the 'unbind blk' command.

Close the file descriptor when unbinding the host block device.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>