KVM: arm64: selftests: Enable tuning of error margin in arch_timer test

There are intermittent failures occurred when stressing the
arch-timer test in a Qemu VM:

 Guest assert failed,  vcpu 0; stage; 4; iter: 3
 ==== Test Assertion Failure ====
   aarch64/arch_timer.c:196: config_iter + 1 == irq_iter
   pid=4048 tid=4049 errno=4 - Interrupted system call
      1  0x000000000040253b: test_vcpu_run at arch_timer.c:248
      2  0x0000ffffb60dd5c7: ?? ??:0
      3  0x0000ffffb6145d1b: ?? ??:0
   0x3 != 0x2 (config_iter + 1 != irq_iter)e

Further test and debug show that the timeout for an interrupt
to arrive do have random high fluctuation, espectially when
testing in an virtual environment.

To alleviate this issue, just expose the timeout value as user
configurable and print some hint message to increase the value
when hitting the failure..

Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Anup Patel <anup@brainfault.org>