[libvirt] [RFC] 5-level paging Support

Hi all, Now both qemu and kvm support 5-level paging. We can start qemu with a "cpu,+la57" to set 57-bit vitrual address space. So VM can be aware that it need to enable 5-level paging. We can also set another "cpu,phys-bits=52" to set the VM physical address space. Actually, VM can still turn on 5 level paging even without "phys-bits=52", yet this means the guest physical address width are limited, meaning less practical benefits. In to support 5-level paging, I suggest to add two attribute for the domain cpu element in libvirt. <cpu la57='yes', phys-bits='52'\> Here we need to be able to set phys-bits directly, because it is potentially migration sensitive. If la57='no', libvirt will ignore phys-bits, no matter whether hardware support la57. If la57='yes', libvirt will probe the host capability, and will throw error if hardware does support la57. If la57='yes', even phys-bits less than 52, libvirt will also pass it to qemu though less practical benefits. [info] [1] [Qemu-devel] [PATCH]x86: implement la57 paging mode <https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg02096.html> [2] [Qemu-devel] [PATCH v4 2/5] x86: Allow physical address bits to be set <https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg01950.html> BR Shaohe Feng