qemu: amend existing table of device weights

qemu: amend existing table of device weights

Prior to this patch, for a running dom, the commands:

$ virsh blkiotune dom --device-weights /dev/sda,502,/dev/sdb,498
$ virsh blkiotune dom --device-weights /dev/sda,503
$ virsh blkiotune dom
weight         : 500
device_weight  : /dev/sda,503

claim that /dev/sdb no longer has a non-default weight, but
directly querying cgroups says otherwise:

$ cat /cgroup/blkio/libvirt/qemu/dom/blkio.weight_device
8:0     503
8:16    498

After this patch, an explicit 0 is required to remove a device path
from the XML, and omitting a device path that was previously
specified leaves that device path untouched in the XML, to match
cgroups behavior.

* src/qemu/qemu_driver.c (parseBlkioWeightDeviceStr): Rename...
(qemuDomainParseDeviceWeightStr): ...and use correct type.
(qemuDomainSetBlkioParameters): After parsing string, modify
rather than replacing existing table.
* tools/virsh.pod (blkiotune): Tweak wording.