fixed graph failure for VM names starting with digits
used "Validate filednames" Python stanza from docs to improve
clean_vm_name functions, helping graph creations for VM with names
starting with digits, like "150-121-Apache".
remember: each data source in a plugin must be identified by a field
name (in this case VM names). The characters must be [a-zA-Z0-9_],
while the first character must be [a-zA-Z_].
The network interface parser of this plugin was overly specific before.
It relied on a specific format of the arguments handed over to kvm while
starting the VM. For example the following format was usable:
... -netdev tap,ifname=foo,...
But kvm/qemu support a variety of ways for configuring network
interfaces via the commandline. E.g. libvirt does not use the "ifname"
parameter above. Thus VMs running on a host controlled via libvirt
cannot be tracked with this plugin.
This limititation is now clearly documented in the header of the plugin.
The old parser seemed to rely on a simple "-name foo" argument format of
kvm/qemu. The changed parser also accepts the following formats:
* name,foo=bar,baz=bot
* guest=name,foo=bar
