vfio/iommufd: Make a separate call to get IOMMU capabilities

vfio/iommufd: Make a separate call to get IOMMU capabilities

Currently we depend on .realize() calling iommufd_backend_get_device_info()
to get IOMMU capabilities and check for dirty page tracking support.

By make a extra separate call, this dependency is removed. This happens
only during device attach, it's not a hot path.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250423072824.3647952-2-zhenzhong.duan@intel.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>