qemu_command: Resolve Coverity RESOURCE_LEAK

In qemuParseISCSIString() if an error was returned, then the call
to qemuParseDriveURIString() where the uri is free'd wouldn't be run

src/qemu/qemu_command.c

@@ -2795,6 +2795,7 @@ qemuParseISCSIString(virDomainDiskDefPtr def)
             virReportError(VIR_ERR_INTERNAL_ERROR,
                            _("invalid name '%s' for iSCSI disk"),
                            def->src->path);
+            virURIFree(uri);
             return -1;
         }
     }