libguestfs.tests: Add test module for libguestfs file operations.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest.utils_misc: Allow null source passed to umount.

Sometimes, we don't know what's source is, but mountpoint
can still be umounted, so allowing to pass null for umount.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>