KVM: x86: Adjust counter sample period after a wrmsr

The sample_period of a counter tracks when that counter will
overflow and set global status/trigger a PMI. However this currently
only gets set when the initial counter is created or when a counter is
resumed; this updates the sample period after a wrmsr so running
counters will accurately reflect their new value.

Signed-off-by: Eric Hankland <ehankland@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>