I guess you want to do that to get some more space for Minix (the Qemu guest machine)?
Do you have the Qemu Launcher installed? If so, then it's gonna be a pretty easy task.
I've successfully tested following steps with DSL as Qemu guest on FreeBSD host:

1. Creating the actual virtual disk on host:
Open a shell and go to a directory, where you want to create the virtual disk and type following command (In my case I wanted the virtual disk to be 1 GB "large")

Code:

qemu-img create virtdisc.img 1G

2. Open the Qemu Launcher, click on the button "Open" next to "Hard disk 1" entry, browse to the directory which is containing the image you have previously created, select the image and click the "Open" button.

3. Launch your Guest OS.

4. Now you need to check if the new created disk is recognized by your Guest OS. Run

Code:

dmesg | grep hd

to know that. If you're lucky, you will find a couple lines similar to these:

Code:

<4>hdb: attached ide-disk driver
<6>hdb: 2097152 sectors (1074 MB) w/256KiB Cache, CHS=130/255/63
<6>Partition check:
<6> hdb: unknown partition table

5. Right now you need to create a partition:

Code:

cfdisk -z /dev/hdb

The usage of this tool is pretty self-explanatory.

6. Reboot Guest OS

7. Check if the partition was successfully created, use "dmesg | grep hd" again. This time the hdb line will look slightly different:

Code:

<6> hdb: hdb1

Looks good ^^

8. It's time to format the partition:

Code:

mkfs /dev/hdb1

9. Mount your new virtual disk:

Code:

mount /dev/hdb1 /mnt/hdb1

10. Enjoy

Code:

df -h