automatically added /boot/efi partition can exceed max primary partitions

[Olivier Gayot]

Steps to reproduce
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* Have a disk with a DOS partition table and 3 existing primary partitions.
* Run the desktop installer under UEFI and navigate to manual partitioning
* Create a new partition and mount it at /

This will result in a crash and the logs will show:
> Exception: Exceeded number of available partitions

Adding the rootfs partition will actually create two partitions:
* a partition mounted at /
* a partition mounted at /boot/efi

There is enough room for one more primary partition, but not 2.