libxl: do not blunder on if bootloader fails (again)

libxl: do not blunder on if bootloader fails (again)

Do not lose the rc value passed to bootloader_callback.  Do not lose
the rc value from the bl when the local disk detach succeeds.

While we're here rationalise the use of bl->rc to make things clearer.
Set it to zero at the start and always update it conditionally; copy
it into bootloader_callback's argument each time.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>