Extend LibvirtConfigGuest to parse guest cpu element info

Add support for LibvirtConfigGuest to parse LibvirtConfigGuestCPU,
parse CPU element 'mode' and 'match' from dom. This is the first
step to improve live migration CPU compare check.

Change-Id: Iab92b38afeade9876be391584e4e41864641134a
Merge "Extend LibvirtConfigGuest to parse guest cpu element info"