KVM: selftests: Convert VMX's PMU capabilities test to printf guest asserts

Convert x86's VMX PMU capabilities test to use printf-based guest asserts.
Opportunstically add a helper to do the WRMSR+assert so as to reduce the
amount of copy+paste needed to spit out debug information.

Link: https://lore.kernel.org/r/20230729003643.1053367-31-seanjc@google.com


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