KVM: PPC: Remove mmio_vsx_tx_sx_enabled in KVM MMIO emulation

Originally PPC KVM MMIO emulation uses only 0~31#(5 bits) for VSR
reg number, and use mmio_vsx_tx_sx_enabled field together for
0~63# VSR regs.

Currently PPC KVM MMIO emulation is reimplemented with analyse_instr()
assistance.  analyse_instr() returns 0~63 for VSR register number, so
it is not necessary to use additional mmio_vsx_tx_sx_enabled field
any more.

This patch extends related reg bits (expand io_gpr to u16 from u8
and use 6 bits for VSR reg#), so that mmio_vsx_tx_sx_enabled can
be removed.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>