qemu: Ignore cmp_legacy CPU flag

qemu: Ignore cmp_legacy CPU flag

Since commit 99a637a8 in qemu 10.0, the way the cmp_legacy flag is
reported changed. The same way as happend with the 'ht' flag in commit
c6bd2dd634208, which was fixed in libvirt since commit ba16113c.

This causes migrations from a hypervisor running a qemu version before
that commit to a hypervisor running qemu after that commit fails
with the following error:
guest CPU doesn't match specification: extra features: cmp_legacy

We can just ignore this flag, just like we did with the 'ht' flag.

Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>