The "osxsave" property was removed from QEMU upstream as it was never actually exposed to the guests.
I expect that your existing guest has this CPU flag encoded in its XML config, as it was a previously supported flag.
Fixing it should be as simple as "virsh edit $guest" as root and delete the mention of "osxsave" feature flag.
Newly provisioned guests shouldn't get given this flag in the first place, only upgraded guests will suffer.
The "osxsave" property was removed from QEMU upstream as it was never actually exposed to the guests.
I expect that your existing guest has this CPU flag encoded in its XML config, as it was a previously supported flag.
Fixing it should be as simple as "virsh edit $guest" as root and delete the mention of "osxsave" feature flag.
Newly provisioned guests shouldn't get given this flag in the first place, only upgraded guests will suffer.