genericarm64: change QB_DEFAULT_FSTYPE to wic.zst

meta-yocto rev 506379cb241e6a865139307e06216a07f737b602
changed the IMAGE_FSTYPES to wic.zst but did not change
QB_DEFAULT_FSTYPE to match.  This is causing the error:
runqemu - ERROR - Failed to find rootfs

Change the QB_DEFAULT_FSTYPE and it works as before (only now runqemu
requires the 'snapshot' parameter).

(From meta-yocto rev: 24857a05b0c2cc70733f245cf9614ef5fb126647)

Signed-off-by: Jon Mason <jon.mason@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-yocto-bsp/conf/machine/genericarm64.conf

@@ -49,7 +49,7 @@ QB_SMP ?= "-smp 4"
 # Boot into U-Boot and let that scan the disk for the next step, don't pass any kernel or filesystem hints
 QB_DEFAULT_BIOS ?= "u-boot.bin"
 QB_DEFAULT_KERNEL ?= "none"
-QB_DEFAULT_FSTYPE ?= "wic"
+QB_DEFAULT_FSTYPE ?= "wic.zst"
 QB_FSINFO ?= "wic:no-kernel-in-fs"
 # Mount the wic rootfs as a virtio block device
 QB_ROOTFS_OPT ?= "-drive id=root,file=@ROOTFS@,if=none,format=raw -device virtio-blk-pci,drive=root"