KVM: x86: Share emulator's common register decoding code

KVM: x86: Share emulator's common register decoding code

Remove all duplicate handling of register operands, including picking
the right register class and fetching it, by extracting a new function
that can be used for both REG and MODRM operands.

Centralize setting op->orig_val = op->val in fetch_register_operand()
as well.

No functional change intended.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Link: https://patch.msgid.link/20251114003633.60689-6-pbonzini@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>