I was trying to sysupgrade -s a 7.8 VMM machine to a 7.9 snapshot, but the
upgrade hangs on boot, and am not quite sure how to proceed.  Is this
something that should work?

I can reset the machine and it comes back fine with the old kernel again.
The host machine is already running a 6.9 snapshot from this week.

switching console to com0 >> OpenBSD/i386 BOOT 3.67 boot> booting
hd0a:/bsd.upgrade: 3316243+1512448+4358152+0+425984 [88+160+28]=0x92d300
entry point at 0x201000 Copyright (c) 1982, 1986, 1989, 1991, 1993 The
Regents of the University of California. All rights reserved. Copyright (c)
1995-2026 OpenBSD. All rights reserved. https://www.OpenBSD.org OpenBSD
7.9-beta (RAMDISK_CD) #269: Wed Mar 25 19:56:53 MDT 2026
deraadt@i386.openbsd.org:/usr/src/sys/arch/i386/compile/RAMDISK_CD real mem
= 3220688896 (3071MB) avail mem = 3151630336 (3005MB) random: good seed
from bootblocks mainbus0 at root bios0 at mainbus0: date 06/23/99, BIOS32
rev. 0 @ 0xfefc2, SMBIOS rev. 2.4 @ 0xf2220 (10 entries) bios0: vendor
SeaBIOS version "1.16.3p1-OpenBSD-vmm" date 01/01/2011 bios0: OpenBSD VMM
acpi at bios0 function 0x0 not configured pcibios at bios0 function 0x1a
not configured bios0: ROM list: 0xef000/0x1000! cpu0 at mainbus0:
(uniprocessor) cpu0: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
("GenuineIntel" 686-class) 85 MHz, 06-3c-03 cpu0:
FPU,V86,DE,PSE,TSC,MSR,PAE,CX8,SEP,PGE,CMOV,PAT,PSE36,CFLUSH,MMX,FXSR,SSE,SSE2,SSE3,PCLMUL,SSSE3,FMA3,CX16,SSE4.1,SSE4.2,MOVBE,POPCNT,AES,XSAVE,AVX,F16C,RDRAND,HV,NXE,PAGE1GB,LONG,LAHF,ABM,ITSC,FSGSBASE,BMI1,AVX2,SMEP,BMI2,ERMS,MD_CLEAR,XSAVEOPT,MELTDOWN
pvbus0 at mainbus0: KVM, OpenBSD pci0 at mainbus0 bus 0: configuration mode
1 (no bios) pchb0 at pci0 dev 0 function 0 "OpenBSD VMM Host" rev 0x00
virtio0 at pci0 dev 1 function 0 "Qumranet Virtio 1.x RNG" rev 0x01 viornd0
at virtio0 virtio0: irq 3 virtio1 at pci0 dev 2 function 0 "Qumranet Virtio
1.x Network" rev 0x01 vio0 at virtio1: 1 queue, address fe:e1:bb:d1:15:a7
virtio1: irq 5 virtio2 at pci0 dev 3 function 0 "Qumranet Virtio 1.x
Storage" rev 0x01 vioblk0 at virtio2 virtio2: irq 6 scsibus0 at vioblk0: 1
targets sd0 at scsibus0 targ 0 lun 0: <VirtIO, Block Device, > sd0:
102400MB, 512 bytes/sector, 209715200 sectors virtio3 at pci0 dev 4
function 0 "OpenBSD VMM Control" rev 0x00 vmmci0 at virtio3 virtio3: irq 7
isa0 at mainbus0 isadma0 at isa0 com0 at isa0 port 0x3f8/8 irq 4: ns8250,
no fifo com0: console pcdisplay0 at isa0 port 0x3d0/16 iomem 0xb8000/32768
wsdisplay0 at pcdisplay0 mux 1: console (80x25, vt100 emulation) npx0 at
isa0 port 0xf0/16: reported by CPUID; using exception 16 softraid0 at root
scsibus1 at softraid0: 256 targets