libxl: print message how to recover from xl cpupool-cpu-remove errors

libxl: print message how to recover from xl cpupool-cpu-remove errors

An error occurring when calling "xl cpupool-cpu-remove" might leave
the system in a state where a cpu is neither completely free nor in
a cpupool. This can easily be repaired by adding the cpu via
"xl cpupool-cpu-add" to the cpupool where it was removed from before.
Print a message telling this the user in case of an error.

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>