hw/i386: check if nvdimm is enabled before plugging

The missing of 'nvdimm' in the machine type option '-M' means NVDIMM
is disabled. QEMU should refuse to plug any NVDIMM device in this case
and report the misconfiguration.

The behavior of NVDIMM on unsupported platform (HW/FW) is vendor
specific. For some vendors, it's undefined and the platform may do
anything. Thus, I think QEMU is free to choose the implementation.
Aborting QEMU (i.e. refusing to boot) is the easiest one.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Message-Id: 20170112110928.GF4621@stefanha-x1.localdomain
Message-Id: 20170111093630.2088-1-stefanha@redhat.com
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
