x86 microvm machine stuck under Xen accelerator

As of v9.0.0-rc4, the x86 isapc/pc/q35 machines are unusable with the Xen accel:

$ qemu-system-x86_64 -M microvm,accel=xen
-> vCPUs not running

Behavior seems similar to using the QTest accel.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information