blockcopy: allow larger buf-size

While qemu definitely caps granularity to 64 MiB, it places no
limits on buf-size.  On a machine beefy enough for lots of
memory, a buf-size larger than 2 GiB is feasible, so we should
pass a 64-bit parameter.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_BLOCK_COPY_BUF_SIZE):
Allow 64 bits.

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