frontend: split add disk dialog to new and attach

Currently, 'Add Virtual Disk' dialog encapsulates two different flows:
create a new disk and attach floating disks (one or multiple).
As these are two separate use cases, it would be more intuitive to split
the current dialog into two: 'New Virtual Disk' and 'Attach Virtual
Disks'.

Change-Id: I3d139311d23e2a488a89d5e8918d37fdf7a1d108
Bug-Url: https://bugzilla.redhat.com/1144848
Signed-off-by: Daniel Erez <derez@redhat.com>