KVM: x86: Don't treat ENTER and LEAVE as branches, because they aren't

KVM: x86: Don't treat ENTER and LEAVE as branches, because they aren't

Remove the IsBranch flag from ENTER and LEAVE in KVM's emulator, as ENTER
and LEAVE are stack operations, not branches.  Add forced emulation of
said instructions to the PMU counters test to prove that KVM diverges from
hardware, and to guard against regressions.

Opportunistically add a missing "1 MOV" to the selftest comment regarding
the number of instructions per loop, which commit 7803339fa929 ("KVM:
selftests: Use data load to trigger LLC references/misses in Intel PMU")
forgot to add.

Fixes: 018d70ffcfec ("KVM: x86: Update vPMCs when retiring branch instructions")
Cc: Jim Mattson <jmattson@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/r/20250919004639.1360453-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>