kvm: selftests: vcpu_setup: set cr4.osfxsr

Guest code may want to call functions that have variable arguments.
To do so, we either need to compile with -mno-sse or enable SSE in
the VCPUs. As it should be pretty safe to turn on the feature, and
-mno-sse would make linking test code with standard libraries
difficult, we choose the feature enabling.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>