KVM: sefltests: Use vm_ioctl() and __vm_ioctl() helpers

Use the recently introduced VM-specific ioctl() helpers instead of open
coding calls to ioctl() just to pretty print the ioctl name.  Keep a few
open coded assertions that provide additional info.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>