libxc: Expose the 1GB pages cpuid flag to guest

libxc: Expose the 1GB pages cpuid flag to guest

If hardware support the 1GB pages, expose the feature to guest by
default. Users don't have to use a 'cpuid= ' option in config fil
e to turn it on.

If guest use shadow mode, the 1GB pages feature will be hidden from
guest, this is done in the function hvm_cpuid(). So the change is
okay for shadow mode case.

Signed-off-by: Liang Li <liang.z.li@intel.com>
Signed-off-by: Yang Zhang <yang.z.zhang@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>