Current QEMU HEAD nvme.c does not compile with the default GCC 5.4
on a Ubuntu 16.04 host:

  hw/block/nvme.c:3242:9: error: ‘result’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
         trace_pci_nvme_getfeat_vwcache(result ? "enabled" : "disabled");
         ^
  hw/block/nvme.c:3150:14: note: ‘result’ was declared here
     uint32_t result;
              ^

Explicitly initialize the result to fix it.

Fixes: aa5e55e3 ("hw/block/nvme: open code for volatile write cache")
Fixes: Coverity CID 1446371
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>