KVM: x86: align the code with kvm_x86_call()

KVM: x86: align the code with kvm_x86_call()

The use of static_call_cond() is essentially the same as static_call() on
x86 (e.g. static_call() now handles a NULL pointer as a NOP), and then the
kvm_x86_call() is added to improve code readability and maintainability
for keeping consistent code style.

Fixes 8d032b683c29 ("KVM: TDX: create/destroy VM structure")

Link: https://lore.kernel.org/all/3916caa1dcd114301a49beafa5030eca396745c1.1679456900.git.jpoimboe@kernel.org/
Link: https://lore.kernel.org/r/20240507133103.15052-3-wei.w.wang@intel.com
Signed-off-by: Jun Miao <jun.miao@intel.com>
Link: https://patch.msgid.link/20260105065423.1870622-1-jun.miao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>