target/i386: do not access beyond the low 128 bits of SSE registers

target/i386: do not access beyond the low 128 bits of SSE registers

The i386 target consolidates all vector registers so that instead of
XMMReg, YMMReg and ZMMReg structs there is a single ZMMReg that can
fit all of SSE, AVX and AVX512.

When TCG copies data from and to the SSE registers, it uses the
full 64-byte width.  This is not a correctness issue because TCG
never lets guest code see beyond the first 128 bits of the ZMM
registers, however it causes uninitialized stack memory to
make it to the CPU's migration stream.

Fix it by only copying the low 16 bytes of the ZMMReg union into
the destination register.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>