vfio: Report an error when the 'dma_max_mappings' limit is reached

The VFIO IOMMU Type1 kernel driver enforces a default IOMMU mapping
limit of 65535, which is configurable via the 'dma_max_mappings'
module parameter. When this limit is reached, QEMU issues a warning
and fails the mapping operation, but allows the VM to continue
running, potentially causing issues later. This scenario occurs with
SEV-SNP guests, which must update all IOMMU mappings during
initialization.

To address this, update vfio_ram_discard_register_listener() to accept
an 'Error **' parameter and propagate the error to the caller. This
change will halt the VM immediately, at init time, with the same error
message.

Additionally, the same behavior will be enforced at runtime. While
this might be considered too brutal, the rarity of this case and the
planned removal of the dma_max_mappings module parameter make it a
reasonable approach.

Cc: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250814153419.1643897-1-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

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