KVM: selftests: exit with 0 status code when tests cannot be run

Right now, skipped tests are returning a failure exit code if /dev/kvm does
not exists.  Consistently return a zero status code so that various scripts
over the interwebs do not complain.  Also return a zero status code if
the KVM_CAP_SYNC_REGS capability is not present, and hardcode in the
test the register kinds that are covered (rather than just using whatever
value of KVM_SYNC_X86_VALID_FIELDS is provided by the kernel headers).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>