Recent kernel detect MS Hyper-V and use VMBUS technology for disk access. It works fine on Hyper-V VMM but kernel mis-detect Virtual-PC on Windows 7. Because there is not VMBUS technology in Virtual-PC, so kernel cannot find root disk and fails to boot. Kernel should detect Virtual-PC than Hyper-V. It is known that Hyper-V has a bios vendor name 'VRTUAL' now but it is easily changable. Patch is as follows: /drivers/ata/ata_piix.c

static int prefer_ms_hyperv = 1;
 module_param(prefer_ms_hyperv, int, 0);
 
 static void piix_ignore_devices_quirk(struct ata_host *host)
 {
 #if IS_ENABLED(CONFIG_HYPERV_STORAGE)
 	static const struct dmi_system_id ignore_hyperv[] = {
 		{
 			/* On Hyper-V hypervisors the disks are exposed on
 			 * both the emulated SATA controller and on the
 			 * paravirtualised drivers.  The CD/DVD devices
 			 * are only exposed on the emulated controller.
 			 * Request we ignore ATA devices on this host.
 			 */
 			.ident = "Hyper-V Virtual Machine",
 			.matches = {
 				DMI_MATCH(DMI_SYS_VENDOR,
 						"Microsoft Corporation"),
 				DMI_MATCH(DMI_PRODUCT_NAME, "Virtual Machine"),
+				/* DMI_MATCH(DMI_BIOS_VENDOR, "VRTUAL"), */
 			},
 		},
 		{ }	/* terminate list */
 	};
+	static const struct dmi_system_id find_virtual_pc[] = {
+		{
+			.ident = "Microsoft Virtual-PC",
+			.matches = {
+				DMI_MATCH(DMI_SYS_VENDOR,
+						"Microsoft Corporation"),
+				DMI_MATCH(DMI_PRODUCT_NAME, "Virtual Machine"),
+				DMI_MATCH(DMI_BIOS_VENDOR, "American Megatrends Inc."),
+			},
+		},
+		{ }	/* terminate list */
+	};
 	const struct dmi_system_id *dmi = dmi_first_match(ignore_hyperv);
 
 	if (dmi && prefer_ms_hyperv) {
-		host->flags |= ATA_HOST_IGNORE_ATA;
-		dev_info(host->dev, "%s detected, ATA device ignore set\n",
-			dmi->ident);
+               if (!dmi_first_match(find_virtual_pc)) {
+			host->flags |= ATA_HOST_IGNORE_ATA;
+			dev_info(host->dev, "%s detected, ATA device ignore set\n",
+				dmi->ident);
+		}
 	}
 #endif
 }

The patch is not tested and just a concept to fix problem. You can escape the problem with kernel option 'ata_piix.prefer_ms_hyperv=0'.