Fix VSIE CSR write emulation (#822)

The VSIE CSR write emulation is broken in latest Spike because
it is allowing only VSSIE bit to be update. This patches fixes
ie_write_mask for VSIE CSR to allow VSTIE, VSSIE, and VSEIE
bits to be updated.

With this patch, we have both Xvisor RISC-V and KVM RISC-V
working again on Spike.

Fixes: 5c1d635c

 ("Convert sie/hie/vsie to csr_t family")
Signed-off-by: Anup Patel <anup.patel@wdc.com>