virttest.libvirt_xml: Add sysinfo xml handler

Add sysinfo_xml module to handle sysinfo xml operation.

Signed-off-by: Wayne Sun <gsun@redhat.com>
virttest.virsh: Add virsh sysinfo command function

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1433 from waynesun09/virsh_sysinfo

virttest: Add sysinfo xml handler and virsh sysinfo command function