Kernel: Find boot device by enumerating devices

Since devices are enumerable and can compute their own name inside the
/dev hierarchy, there is no need to try and parse "root=/dev/xxx" by
hand.

This also makes any block device a candidate for the boot device, which
now includes ramdisk devices, so SerenityOS can now boot diskless too.
The disk image generated for QEMU is suitable, as long as it fits in
memory with room to spare for the rest of the system.

Kernel/Storage/StorageManagement.cpp

@@ -130,43 +130,22 @@ NonnullRefPtrVector<DiskPartition> StorageManagement::enumerate_disk_partitions(
 void StorageManagement::determine_boot_device()
 {
     ASSERT(!m_controllers.is_empty());
-    if (!m_boot_argument.starts_with("/dev/hd")) {
-        klog() << "init_stage2: root filesystem must be on an hard drive";
+    if (m_boot_argument.starts_with("/dev/")) {
+        StringView device_name = m_boot_argument.substring_view(5);
+        Device::for_each([&](Device& device) {
+            if (device.is_block_device()) {
+                auto& block_device = static_cast<BlockDevice&>(device);
+                if (device.device_name() == device_name) {
+                    m_boot_block_device = block_device;
+                }
+            }
+        });
+    }
+
+    if (m_boot_block_device.is_null()) {
+        klog() << "init_stage2: boot device " << m_boot_argument << " not found";
         Processor::halt();
     }
-
-    auto drive_letter = m_boot_argument.substring(strlen("/dev/hd"), m_boot_argument.length() - strlen("/dev/hd"))[0];
-
-    if (drive_letter < 'a' || drive_letter > 'z') {
-        klog() << "init_stage2: root filesystem must be on an hard drive name";
-        Processor::halt();
-    }
-
-    size_t drive_index = (u8)drive_letter - (u8)'a';
-    if (drive_index >= m_storage_devices.size()) {
-        klog() << "init_stage2: invalid selection of hard drive.";
-        Processor::halt();
-    }
-
-    auto& determined_boot_device = m_storage_devices[drive_index];
-    auto root_device = m_boot_argument.substring(strlen("/dev/hda"), m_boot_argument.length() - strlen("/dev/hda"));
-    if (!root_device.length()) {
-        m_boot_block_device = determined_boot_device;
-        return;
-    }
-
-    auto partition_number = root_device.to_uint();
-    if (!partition_number.has_value()) {
-        klog() << "init_stage2: couldn't parse partition number from root kernel parameter";
-        Processor::halt();
-    }
-
-    if (partition_number.value() > determined_boot_device.m_partitions.size()) {
-        klog() << "init_stage2: invalid partition number!";
-        Processor::halt();
-    }
-
-    m_boot_block_device = determined_boot_device.m_partitions[partition_number.value() - 1];
 }
 
 void StorageManagement::determine_boot_device_with_partition_uuid()