Make ubuntu cloud images work with zvols

[dfberlin]

Version 1.5 as in the FreeBSD packages fails to import a Ubuntu 22.04 Cloud image - and most likely others.

sudo vm img
DATASTORE           FILENAME
default             jammy-server-cloudimg-amd64-disk-kvm.img

cat /bhyve/vm/.templates/cloud-zvol-ubuntu.conf 
loader="uefi"
cpu=1
memory=1G
network0_type="virtio-net"
network0_switch="public"
disk0_name="disk0"
disk0_dev="sparse-zvol"
disk0_type="virtio-blk"

The following command lead to an error by qemu-img dd -> device busy.

sudo vm create -t cloud-zvol-ubuntu -s 6G -i jammy-server-cloudimg-amd64-disk-kvm.img -C -n "ip=192.168.xxx.yyy/24;gateway=192.168.xxx.yyy;nameservers=192.168.xxx.yyy" -k ~/falpi_df_id_rsa.pub showcase1

The changes I made repeat the qemu-img command for up to 30 times with a 1 second sleep to allow the zvol to come up and be actually ready to be written on.
Additionally the hard coded disk type in core::create_cloud_init has been changed to virtio-blk, since at least the Ubuntu cloud images require this to have cloud init working.

This is my first ever pull request - so in case there is something wrong, just let me know ;)

Greetings,
Dirk