MAINTAINERS: Add some missing entries related to accelerators

Add some files from accel/stubs/, include/hw/kvm/ and scripts/kvm/
to the MAINTAINERS file.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1542891438-13329-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>