vfio: Move vfio_get_info_dma_avail() into helpers.c

vfio: Move vfio_get_info_dma_avail() into helpers.c

vfio_get_info_dma_avail() is a low level routine similar to the other
routines extracting capabilities from 'struct vfio_iommu_type1_info'.
It belongs to file "helpers.c".

Reviewed-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20250318095415.670319-17-clg@redhat.com
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-18-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>