virgl: resolve to correct level during texture read

When PIPE_TRANSFER_READ requires a resolve, we blit from the host
storage to a temporary storage, and do a format conversion from the
temporary storage to the guest storage.  This change makes sure we
convert to the correct level of the guest storage.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>