virttest.utils_libguestfs: Add functions for file operations.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.libguestfs: Add functions for file operations.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
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>
Merge pull request #1156 from yumingfei/virt_file_oper

Libguestfs: Add testcases for libguestfs file operations.