libvirt_xml.devices: Add slots for channel.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.libvirt: Support discard for creating disk xml.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1784 from yumingfei/discard

libvirt_xml.channel enhancements&Discard configurations