A generic error code was returned, if the user aborted a migration job.
This made it hard to distinguish between a user requested abort and an
error that might have occured. This patch introduces a new error code,
which is returned in the specific case of a user abort, while leaving
all other failures with their existing code. This makes it easier to
distinguish between failure while mirgrating and an user requested
abort.

 * include/libvirt/virterror.h: - add new error code
 * src/util/virterror.c: - add message for the new error code
 * src/qemu/qemu_migration.h: - Emit operation aborted error instead of
                                operation failed, on migration abort