vfio: Make VFIOIOMMUClass::attach_device() and its wrapper return bool

vfio: Make VFIOIOMMUClass::attach_device() and its wrapper return bool

Make VFIOIOMMUClass::attach_device() and its wrapper function
vfio_attach_device() return bool.

This is to follow the coding standand to return bool if 'Error **'
is used to pass error.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>