virgl: add VIRGL_DEBUG_SYNC and VIRGL_DEBUG_XFER
They are useful to identify if a rendering artifact is due to general CPU/GPU sync issue (VIRGL_DEBUG_SYNC) or, more specifically, a transfer sync issue (VIRGL_DEBUG_XFER). For example, the flickering in Unigine Valley mentioned in !1159 (comment 180002) is gone when either of the flags is set.