fix #4627: ui: backup edit: don't deselect all guests on load

fix #4627: ui: backup edit: don't deselect all guests on load

'selectPoolMembers' will be called when the poolid field changes.
(That can even happen when the mode is not even 'pool') Due to how
the fields are set, there is a race condition that this will be
called after the remaining fields were set up, including the VM list
that might have entries selected.

Since the first thing we do here is to deselect all, this wiped the
virtual guest selection sometimes.

To fix it, check if we're actually in the correct mode before doing
anything.

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