libxl: Fix carefd lock leak in save callout

libxl: Fix carefd lock leak in save callout

If libxl_pipe fails we leave the carefd locked, which translates to
the atfork lock remaining held.  This would probably cause the process
to deadlock shortly afterwards.

Of course libxl_pipe is very unlikely to fail unless things are
already going very badly.  This bug has not been observed anywhere as
far as we are aware.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>