drm/i915/gvt/kvmgt: return meaningful error for vgpu creating failure

The vgpu_create() routine we called returns meaningful errors to indicate
failures, so we'd better to pass it to our caller, the mdev framework,
whereby the sysfs is able to tell userspace what happened.

Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>