always pin windows VMs to a machine version by default

always pin windows VMs to a machine version by default

A fix for violating a important standard for booting[0] in recently
packaged QEMU 5.2 surfaced some issues with Windows based VMs in our
forum[1], which seem to be quite sensitive for such changes (it seems
they derive lots of their device assignment from ACPI).
User visible effects are loss of any network configuration due to
windows thinking it was swapped with a new one, and starts with a
fresh config - this is mostly problematic for setups with static
address assignment.

There may be lots of other, more subtle, effects and the PVE admin is
also not always the VM admin, so we really need to avoid such
negative effects. Do this by pinning the version of any windows based
VMs to either the minimum of (5.1, kvm-version) for existing VMs or
the kvm-version at time of VM creation for new ones.

There are patches in pve-manager for user to be able to change the
pinned version themself in the webinterface, so this can now also get
adapted more easily if there surface any other issues (with new or
old version) in the future.

0: https://lists.gnu.org/archive/html/qemu-devel/2021-02/msg08484.html
1: https://forum.proxmox.com/threads/warning-latest-patch-just-broke-all-my-windows-vms-6-3-4-patch-inside.84915/page-2#post-373331

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>