Fix VSSTATUS bits updation (#568)

This patch fixes VSSTATUS bits updations as follows:
1. set_csr() should not allow writs to VSSTATUS.SD bit instead
   VSSTATUS.SD bit should be derived again after VSSTATUS is updated
2. set_virt() should copy FS, VS and XS bits when transitioning
   from virt=on (VS/VU-mode) to virt=off (HS/M-mode)

Signed-off-by: Anup Patel <anup.patel@wdc.com>