accel/kvm: Specify default IPA size for arm64

accel/kvm: Specify default IPA size for arm64

Before this change, the default KVM type, which is used for non-virt
machine models, was 0.

The kernel documentation says:
> On arm64, the physical address size for a VM (IPA Size limit) is
> limited to 40bits by default. The limit can be configured if the host
> supports the extension KVM_CAP_ARM_VM_IPA_SIZE. When supported, use
> KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits) to set the size in the machine type
> identifier, where IPA_Bits is the maximum width of any physical
> address used by the VM. The IPA_Bits is encoded in bits[7-0] of the
> machine type identifier.
>
> e.g, to configure a guest to use 48bit physical address size::
>
>     vm_fd = ioctl(dev_fd, KVM_CREATE_VM, KVM_VM_TYPE_ARM_IPA_SIZE(48));
>
> The requested size (IPA_Bits) must be:
>
>  ==   =========================================================
>   0   Implies default size, 40bits (for backward compatibility)
>   N   Implies N bits, where N is a positive integer such that,
>       32 <= N <= Host_IPA_Limit
>  ==   =========================================================

> Host_IPA_Limit is the maximum possible value for IPA_Bits on the host
> and is dependent on the CPU capability and the kernel configuration.
> The limit can be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the
> KVM_CHECK_EXTENSION ioctl() at run-time.
>
> Creation of the VM will fail if the requested IPA size (whether it is
> implicit or explicit) is unsupported on the host.
https://docs.kernel.org/virt/kvm/api.html#kvm-create-vm

So if Host_IPA_Limit < 40, specifying 0 as the type will fail. This
actually confused libvirt, which uses "none" machine model to probe the
KVM availability, on M2 MacBook Air.

Fix this by using Host_IPA_Limit as the default type when
KVM_CAP_ARM_VM_IPA_SIZE is available.

Cc: qemu-stable@nongnu.org
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-id: 20230727073134.134102-3-akihiko.odaki@daynix.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 1ab445af8cd99343f29032b5944023ad7d8edebf)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>