selftests: kvm: move get_run_delay() into lib/test_util

get_run_delay() is defined static in xen_shinfo_test and steal_time test.
Move it to lib and remove code duplication.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>