qm: add VM import command

qm: add VM import command

Add a command that can be used together with volumes from the new
'import' content type of storage plugins.

For now only the new ESXi exposes that content type, but in the long
run its planned to migrate over the existing OVF/OVA infra and extend
it so that it will replace the 'ovfimport' command.

Originally-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
 [ TL: split out to separate commit and add message, fix completing
   VMID to propose unused ones, note explicitly when in dry-run mode ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>