create-vm: Add extra options for nic and disk

Add parameters to define extra options for network interface and disk.
This is needed to setup a VM which is later used to boot a Xen kernel.
Xen does not understand virtio so to use the VM to boot a Xen kernel,
add e.g. "--disk-opts bus=sata --nic-opts model=e1000".
I meant, drop the consecutive `""`, not the quotes either side of `$disk_opts`:

    --disk path="$vm_disk,format=qcow2,cache=$cache_mode$disk_opts" \

As it stands, this will currently break if invoked with `--disk-opts "foo bar"`.  I know that the disk options probably shouldn't contain a space, but if they do, the wrong error will likely be output from `virt-install`.