KVM: x86: Update MAINTAINTERS to include selftests

Give KVM x86 the same treatment as all other KVM architectures, and
officially take ownership of x86 specific KVM selftests (changes have
been routed through kvm and/or kvm-x86 for quite some time).

Cc: kvm@vger.kernel.org
Link: https://lore.kernel.org/r/20230817234114.1420092-1-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>