KVM: selftests: Convert the nSVM software interrupt test to printf guest asserts

Convert x86's nested SVM software interrupt injection test to use printf-
based guest asserts.  Opportunistically use GUEST_ASSERT() and
GUEST_FAIL() in a few locations to spit out more debug information.

Link: https://lore.kernel.org/r/20230729003643.1053367-28-seanjc@google.com


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