libvirt.tests: Add virsh sysinfo test case

Add virsh sysinfo case and cfg file.

As sysinfo output varies on different platform (ARMv7, s390x, PowerPC,
!WIN32 && x86), only check processor part which each platform sysinfo
output contain, and compare cpu version from processor node xml in
sysinfo with dmidecode output, skip check if dmidecode command fail.

Signed-off-by: Wayne Sun <gsun@redhat.com>