MAINTAINERS: Add KVM selftests to existing KVM entry

It's safe to assume Paolo and Radim are maintaining the KVM selftests
given that the vast majority of commits have their SOBs.  Play nice
with get_maintainers and make it official.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>