hw/arm/virt: KVM: Probe for KVM_CAP_ARM_VM_IPA_SIZE when creating scratch VM

hw/arm/virt: KVM: Probe for KVM_CAP_ARM_VM_IPA_SIZE when creating scratch VM

Although we probe for the IPA limits imposed by KVM (and the hardware)
when computing the memory map, we still use the old style '0' when
creating a scratch VM in kvm_arm_create_scratch_host_vcpu().

On systems that are severely IPA challenged (such as the Apple M1),
this results in a failure as KVM cannot use the default 40bit that
'0' represents.

Instead, probe for the extension and use the reported IPA limit
if available.

Cc: Andrew Jones <drjones@redhat.com>
Cc: Eric Auger <eric.auger@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20210822144441.1290891-2-maz@kernel.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>