storage plugins: add 'import' content type

storage plugins: add 'import' content type

Add a new 'import' content type which will be the corner stone for a
better API and UI integrated way to import virtual guests into Proxmox
VE.

For starters this will be used to implement a ESXi adapter, so that
those VMs can get imported nicely.

Later we want to integrate the OVF/OVA import skeletons we got in
qemu-server to something more usable here.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
 [ TL: add more commit message with some background ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>