blockjob: wire up online qemu block-commit

blockjob: wire up online qemu block-commit

This is the bare minimum to kick off a block commit.  In particular,
flags support is missing (shallow requires us to crawl the backing
chain to determine the file name to pass to the qemu monitor command;
delete requires us to track what needs to be deleted at the time
the completion event fires).  Also, we are relying on qemu to do
error checking (such as validating 'top' and 'base' as being members
of the backing chain), including the fact that the current qemu code
does not support committing the active layer (although it is still
planned to add that before qemu 1.3).  Since the active layer won't
change, we have it easy and do not have to alter the domain XML.
Additionally, this will fail if SELinux is enforcing, because we fail
to grant qemu proper read/write access to the files it will modify.

* src/qemu/qemu_driver.c (qemuDomainBlockCommit): New function.
(qemuDriver): Register it.