vfio/iommufd: Add hw_caps field to HostIOMMUDeviceCaps

vfio/iommufd: Add hw_caps field to HostIOMMUDeviceCaps

Store the value of @caps returned by iommufd_backend_get_device_info()
in a new field HostIOMMUDeviceCaps::hw_caps. Right now the only value is
whether device IOMMU supports dirty tracking (IOMMU_HW_CAP_DIRTY_TRACKING).

This is in preparation for HostIOMMUDevice::realize() being called early
during attach_device().

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>