[PATCH pve-kernel v3 2/2] fix #8027: config: enable CONFIG_HYPERV_VMBUS

[結友 <miagetegorann@gmail.com>]

Linux 6.18 split the Hyper-V guest support options: CONFIG_HYPERV became a
plain bool covering only the core hypervisor support, and the VMBus bus
driver moved to the new tristate CONFIG_HYPERV_VMBUS:

  -obj-$(CONFIG_HYPERV)       += hv_vmbus.o
  +obj-$(CONFIG_HYPERV_VMBUS) += hv_vmbus.o

(upstream 94b04355e639 ("Drivers: hv: Add CONFIG_HYPERV_VMBUS option") and
e3ec97c3abaf ("Drivers: hv: Make CONFIG_HYPERV bool"))

So the `-e CONFIG_HYPERV` added in ef257cb ("config: enable CONFIG_HYPERV")
does not build the VMBus driver into the kernel anymore. CONFIG_HYPERV_VMBUS
does default to HYPERV, but the Ubuntu config sets it to 'm' explicitly and
olddefconfig keeps that. proxmox-kernel-6.17.13-21-pve accordingly still
lists kernel/drivers/hv/hv_vmbus.ko in modules.builtin, while 7.0.2-6-pve
and 7.0.14-25-pve ship it as a module.

That breaks installing Proxmox VE in a Hyper-V Generation 2 VM, where the
storage controller is a VMBus device: VMBus devices only appear in sysfs
once hv_vmbus has been probed. The installer initrd collects the list of
/sys/devices/**/modalias once, before loading the modules discovered
through those aliases. It loads hv_vmbus through the ACPI VMBus node,
but never sees the vmbus: aliases of its children. hv_storvsc therefore
stays unloaded, no /dev/sr0 shows up and the installation aborts with
"no device with valid ISO found". Accordingly the 9.2 ISO fails, while
the 9.1 one with its 6.17 kernel works.

With hv_vmbus built in again, the VMBus children are already enumerated by
the time the initrd runs, so that single modalias scan picks up hv_storvsc
on its own.

Record in the README why it has to be built in rather than modular.

Signed-off-by: 結友 <miagetegorann@gmail.com>
---
 README                            | 5 +++++
 debian/rules.d/config-common.opts | 1 +
 2 files changed, 6 insertions(+)