KVM: Add a simplified wrapper for registering perf callbacks

KVM: Add a simplified wrapper for registering perf callbacks

Add a parameter-less API for registering perf callbacks in anticipation of
introducing another x86-only parameter for handling mediated PMU PMIs.

No functional change intended.

Acked-by: Anup Patel <anup@brainfault.org>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>