libxl: wait for console path before firing console_available

libxl: wait for console path before firing console_available

If the path doesn't become available after LIBXL_INIT_TIMEOUT
seconds, fail the domain creation.

If we skip the bootloader, the TTY path will be set by xenconsoled.
However, there is no guarantee that this will happen by the time we
want to call the console_available callback, so we have to wait.

Signed-off-by: Paweł Marczewski <pawel@invisiblethingslab.com>
Reviewed-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>