x86/acrn: Remove redundant chars from ACRN signature

hypervisor_cpuid_base() only handles 12 chars of the hypervisor
signature string but is provided with 14 chars.

Remove the redundancy. Additionally, replace the user space uint32_t
with preferred kernel type u32.

Signed-off-by: Shuo Liu <shuo.a.liu@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/r/20200806114111.9448-1-shuo.a.liu@intel.com