drm/i915/gvt: fix an issue in emulating cfg space PCI_COMMAND

There's an issue in current cfg space emulation for PCI_COMMAND (offset
0x4): when guest changes some bits other than PCI_COMMAND_MEMORY, this
write operation will not be written to virutal cfg space successfully.

This patch is to fix the wrong behavior above.

Signed-off-by: Min He <min.he@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>