KVM: x86: Bump hypercall stat prior to fully completing hypercall

Increment the "hypercalls" stat for KVM hypercalls as soon as KVM knows
it will skip the guest instruction, i.e. once KVM is committed to emulating
the hypercall.  Waiting until completion adds no known value, and creates a
discrepancy where the stat will be bumped if KVM exits to userspace as a
result of trying to skip the instruction, but not if the hypercall itself
exits.

Handling the stat in common code will also avoid the need for another
helper to dedup code when TDX comes along (TDX needs a separate completion
path due to GPR usage differences).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20241128004344.4072099-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>