This patch adds a new field in hvm to indicate 1gb is supported by CPU. 
In addition, users can turn 1GB feature on/off using a Xen option 
("hap_1gb", default is off).

Signed-off-by: Wei Huang <wei.huang2@amd.com>
Acked-by: Dongxiao Xu <dongxiao.xu@intel.com>