KVM: take over co-maintainership from Marcelo, fix MAINTAINERS entry

As announced last week by Marcelo Tosatti, I will be co-maintaining
KVM together with Gleb.

Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: kvm@vger.kernel.org
Acked-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>