perf: Add APIs to load/put guest mediated PMU context

perf: Add APIs to load/put guest mediated PMU context

Add exported APIs to load/put a guest mediated PMU context.  KVM will
load the guest PMU shortly before VM-Enter, and put the guest PMU shortly
after VM-Exit.

On the perf side of things, schedule out all exclude_guest events when the
guest context is loaded, and schedule them back in when the guest context
is put.  I.e. yield the hardware PMU resources to the guest, by way of KVM.

Note, perf is only responsible for managing host context.  KVM is
responsible for loading/storing guest state to/from hardware.

[sean: shuffle patches around, write changelog]
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Link: https://patch.msgid.link/20251206001720.468579-8-seanjc@google.com