While debugging an unrelated issue, I come to an interesting effect.
When I run qemu with two devices, one vhost-user-fs-pci and another,
say, virtio-blk, - both these devices have bootindex property. If
I specify virtio-blk first and vhost-user-fs-pci second, everything
works as expected and seabios boots the OS out of virtio-blk device.
But if I specify vhost-user-fs-pci first, seabios will try to boot
off virtiofs one, which isn't even a block device, and for which
seabios does not have any driver. And in this case the boot just
Booting from Hard Disk..
Boot failed: could not read the boot disk
next it tries pxe boot, cdrom, and fails.
Does it make sense to try to boot off unsupported devices?