Prepare accounting for disk sizes of non-boot disks

[dlucredativ]

This change is a preparation to closing issue #115. ProxLB now gathers all migration-relevant disk metrics for further decision making. One such instance of decision making is planned via credativ/proxlb-solver#13.

This PR does not change disk-based decisions in the legacy (non-proxlb-solver) migration logic.

[dlucredativ]

Example topology:

2026-07-10 17:54:51,506 - ProxLB - DEBUG - Storage topology collected: {'local': Storage(name='local', type='dir', shared=False, content=['backup', 'import', 'iso', 'vztmpl'], nodes={'prox-dev-host-172-26-250-156': NodeStatus(total=15808331776, used=4640489472, avail=11167842304, active=True, enabled=True), 'prox-dev-host-172-26-250-152': NodeStatus(total=15808331776, used=5434716160, avail=10373615616, active=True, enabled=True), 'prox-dev-host-172-26-250-154': NodeStatus(total=15808331776, used=5634625536, avail=10173706240, active=True, enabled=True)}, guest_disks={}), 'local-156': Storage(name='local-156', type='dir', shared=False, content=['rootdir', 'images'], nodes={'prox-dev-host-172-26-250-156': NodeStatus(total=15808331776, used=4640489472, avail=11167842304, active=True, enabled=True), 'prox-dev-host-172-26-250-152': NodeStatus(total=0, used=0, avail=0, active=False, enabled=False), 'prox-dev-host-172-26-250-154': NodeStatus(total=0, used=0, avail=0, active=False, enabled=False)}, guest_disks={102: GuestDisks(allocated=2097152, provisioned=805306368)}), 'local-154': Storage(name='local-154', type='dir', shared=False, content=['images', 'rootdir'], nodes={'prox-dev-host-172-26-250-156': NodeStatus(total=0, used=0, avail=0, active=False, enabled=False), 'prox-dev-host-172-26-250-152': NodeStatus(total=0, used=0, avail=0, active=False, enabled=False), 'prox-dev-host-172-26-250-154': NodeStatus(total=15808331776, used=5634625536, avail=10173706240, active=True, enabled=True)}, guest_disks={100: GuestDisks(allocated=2097152, provisioned=805306368), 109: GuestDisks(allocated=860893184, provisioned=8589934592)}), 'local-lvm': Storage(name='local-lvm', type='lvmthin', shared=False, content=['rootdir', 'images'], nodes={'prox-dev-host-172-26-250-156': NodeStatus(total=13723762688, used=0, avail=13723762688, active=True, enabled=True), 'prox-dev-host-172-26-250-152': NodeStatus(total=13723762688, used=0, avail=13723762688, active=True, enabled=True), 'prox-dev-host-172-26-250-154': NodeStatus(total=13723762688, used=0, avail=13723762688, active=True, enabled=True)}, guest_disks={}), 'local-152': Storage(name='local-152', type='dir', shared=False, content=['images', 'rootdir'], nodes={'prox-dev-host-172-26-250-156': NodeStatus(total=0, used=0, avail=0, active=False, enabled=False), 'prox-dev-host-172-26-250-152': NodeStatus(total=15808331776, used=5434716160, avail=10373615616, active=True, enabled=True), 'prox-dev-host-172-26-250-154': NodeStatus(total=0, used=0, avail=0, active=False, enabled=False)}, guest_disks={104: GuestDisks(allocated=1048576, provisioned=536870912), 105: GuestDisks(allocated=1048576, provisioned=268435456)})}