blockcopy: add qemu implementation of new API

blockcopy: add qemu implementation of new API

The hard part of managing the disk copy is already coded; all
this had to do was convert the XML and virTypedParameters into
the internal representation.

With this patch, all blockcopy operations that used the old
API should also work via the new API.  Additional extensions,
such as supporting the granularity tunable or a network rather
than file destination, will be added as later patches.

* src/qemu/qemu_driver.c (qemuDomainBlockCopy): New function.

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