ui: guest import: rework to tab panels

ui: guest import: rework to tab panels

instead of putting all options on a single panel, split into 'general'
and 'advanced'

we move all disk/cd/network related grids to the advanced tab, and
introduce a 'default storage' and 'default bridge' selector to the
general tab.

this way, one can import very quickly when the default are fine or
when only the storage/bridge should change, but can still set the
storage/bridges etc. very fine grained.

also we have the possibility to extend the window in the future with
more tabs.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>