vzdump: error out for master-key backup but no QEMU support

vzdump: error out for master-key backup but no QEMU support

Our QEMU gained master-key support for Proxmox VE 6.4 with initial
QEMU 5.2.0 packaging in 0b8da68 ("add PBS master key support")
version.
As we're now two major releases in the future any VM needs to run
with a newer QEMU version we can just make this a hard-error, as
there really should be no use-case left. After all we only support
upgrading directly to the next major release, so one needs to do at
least a migration (or shutdown) of the VM to reboot the node for
upgrading to Proxmox VE 8, so the lowest QEMU version baseline is 6.0
for Proxmox VE 8 (i.e., the version from PVE 7.0).

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