To add add disk to a machine, go to deployment page and open a vm configuration (Edit config)
copy existing disk tag and pest it below the same.
Example:
Existing disk tag...
<disk device='disk' type='file'>
<source file='/opt/infracc/managed-host/deploycache/deploy/admin/fd2dc792-f485/__top__.1.qcow2' />
<target bus='ide' dev='hda' />
<driver name='qemu' type='qcow2' />
</disk>
New disk tag...
<disk device='disk' type='file'>
<source file='/opt/infracc/managed-host/deploycache/deploy/admin/fd2dc792-f485/disk1.img' />
<target bus='ide' dev='hdb' />
<driver name='qemu' type='qcow2' />
</disk>
As show in New disk tag, disk1.img is the new disk image, that needs to be create at same soruce path as of existing old tag. i.e /opt/infracc/managed-host/deploycache/deploy/admin/fd2dc792-f485/
go to managed host on which vm is deployed.
cd to path source file path
cd /opt/infracc/managed-host/deploycache/deploy/admin/fd2dc792-f485/
Create disk file 'disk1.img' using command.
sudo bash
qemu-img create -f qcow2 disk1.img 50G
chmod 700 disk1.img
Make sure to change 'dev' to hdb or hdc depending on number of disk.
save changes to Edit config, and start virtual machine.