Meta+Documentation: Don't rebuild disk image for every run on OpenBSD

Documentation/BuildInstructionsOther.md

@@ -88,7 +88,7 @@ apk add cmake e2fsprogs grub-bios samurai mpc1-dev mpfr-dev gmp-dev ccache rsync
 ## OpenBSD prerequisites
 
 ```console
-doas pkg_add bash cmake g++ gcc git gmake gmp ninja ccache rsync coreutils qemu sudo
+doas pkg_add bash cmake g++ gcc git gmake gmp ninja ccache rsync coreutils qemu sudo e2fsprogs
 ```
 
 ## FreeBSD prerequisites

Meta/build-image-qemu.sh

@@ -118,6 +118,7 @@ use_genext2fs=0
 if [ "$(uname -s)" = "Darwin" ]; then
   mount_cmd="fuse-ext2 _disk_image mnt -o rw+,allow_other,uid=501,gid=20"
 elif [ "$(uname -s)" = "OpenBSD" ]; then
+  VND=$(vnconfig _disk_image)
   mount_cmd="mount -t ext2fs "/dev/${VND}i" mnt/"
 elif [ "$(uname -s)" = "FreeBSD" ]; then
   MD=$(mdconfig _disk_image)