system/iommufd: Use uint64_t type for IOVA mapping size

system/iommufd: Use uint64_t type for IOVA mapping size

The 'ram_addr_t' type is described as:

  a QEMU internal address space that maps guest RAM physical
  addresses into an intermediate address space that can map
  to host virtual address spaces.

This doesn't represent well an IOVA mapping size. Simply use
the uint64_t type.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250930123528.42878-2-philmd@linaro.org
Signed-off-by: Cédric Le Goater <clg@redhat.com>