x86/perf/core: Add pebs_capable to store valid PEBS_COUNTER_MASK value

The value of pebs_counter_mask will be accessed frequently
for repeated use in the intel_guest_get_msrs(). So it can be
optimized instead of endlessly mucking about with branches.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Message-Id: <20220411101946.20262-7-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>