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>