Commit 89b6284f made it possible to pass either a source name or
the target device to most API demanding a disk designation, but
forgot to update the documentation.  It also failed to update
virDomainBlockStats to take both forms. This patch fixes both the
documentation and the remaining function.

Xen continues to use just device shorthand (that is, I did not
implement path lookup there, since xen does not track a domain_conf
to quickly tie a path back to the device shorthand).

* src/libvirt.c (virDomainBlockStats, virDomainBlockStatsFlags)
(virDomainGetBlockInfo, virDomainBlockPeek)
(virDomainBlockJobAbort, virDomainGetBlockJobInfo)
(virDomainBlockJobSetSpeed, virDomainBlockPull): Document
acceptable disk naming conventions.
* src/qemu/qemu_driver.c (qemuDomainBlockStats)
(qemuDomainBlockStatsFlags): Allow lookup by source name.
* src/test/test_driver.c (testDomainBlockStats): Likewise.