throw error message in elsp emulation handler basing on execlist
submit result. guest will trigger tdr process for recovering, gvt
just follow guest's desire.

v2: populate error to top of mmio emulation logic, comments from
zhenyu
Signed-off-by: Bing Niu <bing.niu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>