KVM: x86: never specify a sample period for virtualized in_tx_cp counters

pmc_reprogram_counter() always sets a sample period based on the value of
pmc->counter. However, hsw_hw_config() rejects sample periods less than
2^31 - 1. So for example, if a KVM guest does

    struct perf_event_attr attr;
    memset(&attr, 0, sizeof(attr));
    attr.type = PERF_TYPE_RAW;
    attr.size = sizeof(attr);
    attr.config = 0x2005101c4; // conditional branches retired IN_TXCP
    attr.sample_period = 0;
    int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
    ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
    ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

the guest kernel counts some conditional branch events, then updates the
virtual PMU register with a nonzero count. The host reaches
pmc_reprogram_counter() with nonzero pmc->counter, triggers EOPNOTSUPP
in hsw_hw_config(), prints "kvm_pmu: event creation failed" in
pmc_reprogram_counter(), and silently (from the guest's point of view) stops
counting events.

We fix event counting by forcing attr.sample_period to always be zero for
in_tx_cp counters. Sampling doesn't work, but it already didn't work and
can't be fixed without major changes to the approach in hsw_hw_config().

Signed-off-by: Robert O'Callahan <robert@ocallahan.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>