KVM: selftests: use unified time type for comparison

With test case kvm_page_table_test, start time is acquired with
time type CLOCK_MONOTONIC_RAW, however end time in timespec_elapsed()
is acquired with time type CLOCK_MONOTONIC. This can cause inaccurate
elapsed time calculation due to mixing timebases, e.g. LoongArch in
particular will see weirdness.

Modify kvm_page_table_test to use unified time type CLOCK_MONOTONIC for
start time.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20230731022405.854884-1-maobibo@loongson.cn


[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>