x86/virt: Add refcounting of VMX/SVM usage to support multiple in-kernel users

Implement a per-CPU refcounting scheme so that "users" of hardware
virtualization, e.g. KVM and the future TDX code, can co-exist without
pulling the rug out from under each other.  E.g. if KVM were to disable
VMX on module unload or when the last KVM VM was destroyed, SEAMCALLs from
the TDX subsystem would #UD and panic the kernel.

Disable preemption in the get/put APIs to ensure virtualization is fully
enabled/disabled before returning to the caller.  E.g. if the task were
preempted after a 0=>1 transition, the new task would see a 1=>2 and thus
return without enabling virtualization.  Explicitly disable preemption
instead of requiring the caller to do so, because the need to disable
preemption is an artifact of the implementation.  E.g. from KVM's
perspective there is no _need_ to disable preemption as KVM guarantees the
pCPU on which it is running is stable (but preemption is enabled).

Opportunistically abstract away SVM vs. VMX in the public APIs by using
X86_FEATURE_{SVM,VMX} to communicate what technology the caller wants to
enable and use.

Cc: Xu Yilun <yilun.xu@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Sagi Shahar <sagis@google.com>
Link: https://patch.msgid.link/20260214012702.2368778-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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