KVM: x86: Add AVX support to the emulator's register fetch and writeback

KVM: x86: Add AVX support to the emulator's register fetch and writeback

Prepare struct operand for hosting AVX registers.  Remove the
existing, incomplete code that placed the Avx flag in the operand
alignment field, and repurpose the name for a separate bit that
indicates:

- after decode, whether an instruction supports the VEX prefix;

- before writeback, that the instruction did have the VEX prefix and
therefore 1) it can have op_bytes == 32; 2) t should clear high
bytes of XMM registers.

Right now the bit will never be set and the patch has no intended
functional change.  However, this is actually more vexing than the
decoder changes itself, and therefore worth separating.

Co-developed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://patch.msgid.link/20251114003633.60689-8-pbonzini@redhat.com
[sean: guard ymm[8-15] accesses with #ifdef CONFIG_X86_64]
Signed-off-by: Sean Christopherson <seanjc@google.com>