x86: Fix a typo in svm_handle_exit()

An exit code the function handles was not the one that is
intercepted.
x86: Always use decode assists for CR0 writes

Temporarily disabled the branch responsible for emulating CR0 writes
completely in software. The reason is that x86_parse_mov_cr() doesn't work
for unpaged guests yet, and that nested SVM seems to fill exitinfos for CR0
intercepts.
x86: Fix handle a guest setting EFER

Guests are not allowed to clear EFER.SVME, so we unconditionally add
this flag to any guest EFER update.