blockjob: implement shallow commit flag in qemu

blockjob: implement shallow commit flag in qemu

Now that we can crawl the chain of backing files, we can do
argument validation and implement the 'shallow' flag.  In
testing this, I discovered that it can be handy to pass the
shallow flag and an explicit base, as a means of validating
that the base is indeed the file we expected.

* src/qemu/qemu_driver.c (qemuDomainBlockCommit): Crawl through
chain to implement shallow flag.
* src/libvirt.c (virDomainBlockCommit): Relax API.