storage: cache backing chain while qemu domain is live

storage: cache backing chain while qemu domain is live

Technically, we should not be re-probing any file that qemu might
be currently writing to.  As such, we should cache the backing
file chain prior to starting qemu.  This patch adds the cache,
but does not use it until the next patch.

Ultimately, we want to also store the chain in domain XML, so that
it is remembered across libvirtd restarts, and so that the only
kosher way to modify the backing chain of an offline domain will be
through libvirt API calls, but we aren't there yet.  So for now, we
merely invalidate the cache any time we do a live operation that
alters the chain (block-pull, block-commit, external disk snapshot),
as well as tear down the cache when the domain is not running.

* src/conf/domain_conf.h (_virDomainDiskDef): New field.
* src/conf/domain_conf.c (virDomainDiskDefFree): Clean new field.
* src/qemu/qemu_domain.h (qemuDomainDetermineDiskChain): New
prototype.
* src/qemu/qemu_domain.c (qemuDomainDetermineDiskChain): New
function.
* src/qemu/qemu_driver.c (qemuDomainAttachDeviceDiskLive)
(qemuDomainChangeDiskMediaLive): Pre-populate chain.
(qemuDomainSnapshotCreateSingleDiskActive): Uncache chain before
snapshot.
* src/qemu/qemu_process.c (qemuProcessHandleBlockJob): Update
chain after block pull.