backends/iommufd: Make iommufd_backend_*() return bool

backends/iommufd: Make iommufd_backend_*() return bool

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

The changed functions include:

iommufd_backend_connect
iommufd_backend_alloc_ioas

By this chance, simplify the functions a bit by avoiding duplicate
recordings, e.g., log through either error interface or trace, not
both.

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>