fix #3580: plugins: make preallocation mode selectable for qcow2 and raw images

fix #3580: plugins: make preallocation mode selectable for qcow2 and raw images

the plugins for file based storages
 * BTRFS
 * CIFS
 * Dir
 * Glusterfs
 * NFS
now allow the option 'preallocation'.

'preallocation' can have four values:
 * default
 * off
 * metadata
 * falloc
 * full
see man pages for `qemu-img` for what these mean exactly. [0]

the defualt value was chosen to be
 * qcow2: metadata (as previously)
 * raw: off

when using 'metadata' as preallocation mode, for raw images 'off'
is used.

[0] https://qemu.readthedocs.io/en/latest/system/images.html#disk-image-file-formats

Signed-off-by: Lorenz Stechauner <l.stechauner@proxmox.com>
Reviewed-by: Fabian Ebner <f.ebner@proxmox.com>
Tested-by: Fabian Ebner <f.ebner@proxmox.com>