qemu: x86-64: Specify CPU model.

Newer versions of qemu need this when used without kvm. The CPU
selection used here (Skylake-Server-v1,+xsave) should not be a problem.
1 job for fix-qemu-cpu-error-when-no-kvm in 11 minutes and 46 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
failed #29203
img

00:11:46

 
Name Stage Failure
failed
img Test
mv "target/x86_64-unknown-redox"/openlibm.partial "target/x86_64-unknown-redox"/openlibm
touch "target/x86_64-unknown-redox"/openlibm
make[2]: Leaving directory '/builds/microcolonel/redox/relibc'
mk/prefix.mk:19: recipe for target 'prefix/x86_64-unknown-redox/relibc-install' failed
make[1]: *** [prefix/x86_64-unknown-redox/relibc-install] Error 2
make[1]: Leaving directory '/builds/microcolonel/redox'
Makefile:68: recipe for target 'ci-img' failed
make: *** [ci-img] Error 2
ERROR: Job failed: exit code 1