KVM: selftests: Convert aarch_timer to printf style GUEST_ASSERT

Convert ARM's aarch_timer test to use printf-based GUEST_ASSERT().
To maintain existing functionality, manually print the host information,
e.g. stage and iteration, to stderr prior to reporting the guest assert.

Link: https://lore.kernel.org/r/20230729003643.1053367-11-seanjc@google.com


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