i386: Add Icelake-Server-v6 CPU model with 5-level EPT support

Windows 11 with WSL2 enabled (Hyper-V) fails to boot with Icelake-Server
{-v5} CPU model but boots well with '-cpu host'. Apparently, it expects
5-level paging and 5-level EPT support to come in pair but QEMU's
Icelake-Server CPU model lacks the later. Introduce 'Icelake-Server-v6'
CPU model with 'vmx-page-walk-5' enabled by default.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220221145316.576138-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>