KVM: selftests: Refactor LBR_FMT test to avoid use of separate macro

Rework the LBR format test to use the bitfield instead of a separate
mask macro, mainly so that adding a nearly-identical PEBS format test
doesn't have to copy-paste-tweak the macro too.

No functional change intended.

Link: https://lore.kernel.org/r/20230311004618.920745-20-seanjc@google.com


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