MAINTAINERS: update mail address for NVDIMM

My Intel mail account will be disabled soon, update the mail info
to my private mail

Signed-off-by: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Message-Id: <1490074437-17059-1-git-send-email-guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>