ui: guest import: use view-model for unique-mac-addr state

ui: guest import: use view-model for unique-mac-addr state

Avoid some global widget config to store internal state, rather use
the existing viewModel for that.

Also avoid unnecessary references and switch to a boxLabel, drop also
the tooltip, as that doesn't add much here where the effect can be
seen in the grid.

Avoid reconfiguring all grids if only the network one can have changed
due to a change of this check box value.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>