KVM: selftests: Don't require THP to run tests

Unless we want to test with THP, then we shouldn't require it to be
configured by the host kernel. Unfortunately, even advising with
MADV_NOHUGEPAGE does require it, so check for THP first in order
to avoid madvise failing with EINVAL.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20201029201703.102716-2-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>