perf: Stop pretending that perf can handle multiple guest callbacks

Drop the 'int' return value from the perf (un)register callbacks helpers
and stop pretending perf can support multiple callbacks.  The 'int'
returns are not future proofing anything as none of the callers take
action on an error.  It's also not obvious that there will ever be
co-tenant hypervisors, and if there are, that allowing multiple callbacks
to be registered is desirable or even correct.

Opportunistically rename callbacks=>cbs in the affected declarations to
match their definitions.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20211111020738.2512932-5-seanjc@google.com