util_test.libguestfs: Support choosing disk or domain in VirtTools.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs.tests: Supplements for guestmount test.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1157 from yumingfei/enhance_guestmount

Libguestfs: supplements of guestmount test.