KVM: x86: Do kvm_cpuid_array capacity checks in terminal functions

Perform the capacity checks on the userspace provided kvm_cpuid_array
in the lower __do_cpuid_func() and __do_cpuid_func_emulated().
Pre-checking the array in do_cpuid_func() no longer adds value now that
__do_cpuid_func() has been trimmed down to size, i.e. doesn't invoke a
big pile of retpolined functions before doing anything useful.

Note, __do_cpuid_func() already checks the array capacity via
do_host_cpuid(), "moving" the check to __do_cpuid_func() simply means
removing a WARN_ON().

Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>