Name the upstream cause of the Debian 13 BIOS hang: Debian bug #1111240

The known-gaps entry, decision log, sample comment and builder comment
blamed the host; the cause is trixie's GRUB config ("terminal_output
gfxterm serial" with a serial console and no video device), reported
and reproduced upstream and still open. UEFI remains the default for the
reason recorded; bookworm images are noted as unaffected.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/provider/kvm/domainxml.go

@@ -37,8 +37,10 @@ type domainParams struct {
 //
 // UEFI is expressed as <os firmware="efi"> with secure boot off, and libvirt
 // picks the OVMF binary and creates the NVRAM file itself (removed again by
-// the nvram undefine flag). Debian 13's BIOS boot path was found to hang
-// after GRUB on a real host, while the same image boots under UEFI.
+// the nvram undefine flag). Under SeaBIOS, Debian 13's GRUB hangs on a
+// headless domain with a serial console (Debian bug #1111240:
+// "terminal_output gfxterm serial" with no video device); the same image
+// boots under UEFI with the serial console intact.
 func buildDomainXML(p domainParams) (string, error) {
 	os := osXML{
 		Type: osTypeXML{Arch: "x86_64", Machine: "q35", Value: "hvm"},