KVM: xen: Use static_call() for invoking kvm_x86_ops hooks

Use static_call() for invoking kvm_x86_ops function that already have a
defined static call, mostly as a step toward having _all_ calls to
kvm_x86_ops route through a static_call() in order to simplify auditing,
e.g. via grep, that all functions have an entry in kvm-x86-ops.h, but
also because there's no reason not to use a static_call().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220128005208.4008533-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>