migration: fix calculation of bandwith limit for non-disk migration

migration: fix calculation of bandwith limit for non-disk migration

The case with:
1. no generic 'migration' limit from the storage plugin
2. a migrate_speed limit in the VM config
was broken. It would assign 0 to migrate_speed when picking the minimum value
and then default to the default value. Fix it by checking if bwlimit is 0
before picking the minimum.

Also, make it a bit more readable by avoiding the trick of //-assigning bwlimit
before the units match up and relying on getting back the original bwlimit value
as the minimum. Instead, only ||-assign after the units match up and don't rely
on other things.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>