x86/pmu: Test PMU virtualization on emulated instructions

Add tests of "instructions retired" and "branch instructions retired,"
to ensure that these events count emulated instructions.

Signed-off-by: Eric Hankland <ehankland@google.com>
[jmattson:
  - Added command-line parameter to conditionally run the new tests.
  - Added pmu-emulation test to unittests.cfg
]
Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20211112235652.1127814-1-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>