Infer boot device for reused LVM and RAID

[ancorgs]

Problem

Sometimes the Agama configuration provided by the user requires Agama to calculate the boot-related partitions while omitting the boot device and setting the "/" filesystem in an MD RAID or an LVM device.

The situation is correctly handled by ConfigSolvers::Boot (it determines what should be the booting device) if:

• the mdRaid entry used for "/" represents a new device to be created
• the mdRaid entry used for "/" points to an existing MD RAID that is considered as bootable (which currently basically means a BIOS RAID)
• the volumeGroup entry used for "/" represents a new device to be created

But it fails (no boot device is calculated) if:

• the mdRaid entry used for "/" points to an existing MD RAID that is not considered as bootable (which currently basically means a software RAID)
• the volumeGroup entry used for "/" points to an existing LVM volume group

Solution

This extends ConfigSolvers::Boot with the ability to determine a candidate boot device in those situations, even adding the corresponding entry to the configuration if needed.

Testing

• Added new unit tests
• Tested manually

[joseivanlopez]

LGTM