drm/i915/gvt: Fix guest boot warning

Simulate MIA core in reset status once GUC engine is reset.

v2: 1. use vgpu_vreg_t() function,
    2. clear MIA_IN_RESET after reading. (Zhenyu)
v3: add comments. (Zhenyu)

Signed-off-by: Gao Fred <fred.gao@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191216160255.29499-1-fred.gao@intel.com