KVM: selftests: Convert steal_time test to printf style GUEST_ASSERT

Convert the steal_time test to use printf-based GUEST_ASERT.
Opportunistically use GUEST_ASSERT_EQ() and GUEST_ASSERT_NE() so that the
test spits out debug information on failure.

Link: https://lore.kernel.org/r/20230729003643.1053367-20-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>