KVM: selftests: Rework macros in PMU counters test to prep for multi-insn loop

Tweak the macros in the PMU counters test to prepare for moving the
CLFLUSH+MFENCE instructions into the loop body, to fix an issue where
a single CLFUSH doesn't guarantee an LLC miss.

Link: https://lore.kernel.org/r/20240628005558.3835480-2-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>