KVM: arm64: Use standard seq_file iterator for vgic-debug debugfs

KVM: arm64: Use standard seq_file iterator for vgic-debug debugfs

The current implementation uses `vgic_state_iter` in `struct
vgic_dist` to track the sequence position. This effectively makes the
iterator shared across all open file descriptors for the VM.

This approach has significant drawbacks:
- It enforces mutual exclusion, preventing concurrent reads of the
  debugfs file (returning -EBUSY).
- It relies on storing transient iterator state in the long-lived
  VM structure (`vgic_dist`).

Refactor the implementation to use the standard `seq_file` iterator.
Instead of storing state in `kvm_arch`, rely on the `pos` argument
passed to the `start` and `next` callbacks, which tracks the logical
index specific to the file descriptor.

This change enables concurrent access and eliminates the
`vgic_state_iter` field from `struct vgic_dist`.

Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260202085721.3954942-4-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>