libxl: adjust point of backend name resolution

libxl: adjust point of backend name resolution

Resolution of a backend name to a domid needs to happen a little earlier
in some cases.

For example, if a domU is specified as a backend for a
disk and, as previously written, libxl__device_disk_setdefault() calls
libxl__resolve_domid() last, then disk->backend_domid still equals
LIBXL_TOOLSTACK_DOMID when libxl__device_disk_set_backend() is called.
This results in libxl__device_disk_set_backend() making an incorrect
attempt to validate the target by calling stat() on a file on dom0,
resulting in ERROR_INVAL (see libxl_device.c lines 239-248), which
prevents creation of the frontend domain.

Likewise, libxl__device_nic_setdefault() previously made use of
nic->backend_domid before it was set.

Signed-off-by: Eric Shelton <eshelton@pobox.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>