An interesting problem I ran into recently: qcontroller had been loading Ubuntu 22.04-25.04 images just fine, and then all of a sudden it stopped working with the 26.04 images. The guest VM crashed with:

[ 0.395796] /dev/root: Can't open blockdev

[ 0.396053] VFS: Cannot open root device "PARTUUID=8bb858b9-78e0-48dc-9232-3d2e823ef748" or unknown-block(0,0): error -6

[ 0.396640] Please append a correct "root=" boot option; here are the available partitions:

[ 0.397116] fd00 368 vda