Add methods to get host/VM cpu topolopy

1. Add accessors for cpu_topolopy in capability xml.
2. Add three func for libvirt VM to get domian cpu topolopy from
different ways.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1801 from will-Do/cpu_management

Add methods to get host/VM cpu topolopy