Move the entirety of XSETBV emulation to x86.c, and assign the
function directly to both VMX's and SVM's exit handlers, i.e. drop the
unnecessary trampolines.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210205005750.3841462-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>