blockjob: avoid compiler uncertainty in info sizing

blockjob: avoid compiler uncertainty in info sizing

We have a policy of avoiding enum types in structs in our public
API, because it is possible for a client to choose compiler options
that can change the in-memory ABI of that struct based on whether
the enum value occupies an int or a minimal size.  But we missed
this for virDomainBlockJobInfo.  We got lucky on little-endian
machines - if the enum fits minimal size (a char), we still end
up padding to the next long before the next field; but on
big-endian, a client interpreting the enum as a char would always
see 0 when the server supplies contents as an int.

* include/libvirt/libvirt.h.in (virDomainBlockJobInfo): Enforce
particular sizing.

Signed-off-by: Eric Blake <eblake@redhat.com>