KVM: selftests: Fix kvm_check_cap() assertion

KVM_CHECK_EXTENSION ioctl can return any negative value on error,
and not necessarily -1. Change the assertion to reflect that.

Signed-off-by: Fuad Tabba <tabba@google.com>
Message-Id: <20210615150443.1183365-1-tabba@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>