fix device RO check

Summary:
- mark deviceNode const
- copy it as deviceName
- mutate deviceName by removing `/dev/`
- use the new deviceName for the read-only check

this makes sure we emit the unmodified deviceNode (e.g. /dev/vda) rather
than the name (e.g. vda). unbreaks for example calamares which doesn't
know what to do with "vda".

CCBUG: 378607

Test Plan:
- without fix clamares fails to list devices
- with the fix calamares lists devices

Reviewers: stikonas

Reviewed By: stikonas

Differential Revision: https://phabricator.kde.org/D6414

src/plugins/libparted/libpartedbackend.cpp

@@ -429,9 +429,11 @@ QList<Device*> LibPartedBackend::scanDevices(bool excludeReadOnly)
         const QStringList output=cmd.output().split(QStringLiteral("\n")).filter(QRegularExpression(QStringLiteral("^disk ")));
         quint32 totalDevices = output.length();
         for (quint32 i = 0; i < totalDevices; ++i) {
-            QString deviceNode = output[i].split(QStringLiteral(" ")).last();
+            const QString deviceNode = output[i].split(QStringLiteral(" ")).last();
             if (excludeReadOnly) {
-                QFile f(QStringLiteral("/sys/block/%1/ro").arg(deviceNode.remove(QStringLiteral("/dev/"))));
+                auto deviceName = deviceNode;
+                deviceName.remove(QStringLiteral("/dev/"));
+                QFile f(QStringLiteral("/sys/block/%1/ro").arg(deviceName));
                 if (f.open(QIODevice::ReadOnly))
                     if (f.readLine().trimmed().toInt() == 1)
                         continue;