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_].
* works on debian bullseye with this modification.
* removed all codestyle issues, checked with "pycodestyle" on debian bullseye, besides two "line too long"
* "config" param must not have newlines to avoid a warning in munin-update.log
On Debian Stretch the cmdline is something like:
qemu-system-x86_64-enable-kvm-nameguest=vmname,debug-threads=on-S-[…]
Without null characters:
qemu-system-x86_64 -enable-kvm -name guest=vmname,debug-threads=on[…]
We need to match only guest=vmname, so the regex
"^.*guest=([a-zA-Z0-9.-_-]*).*$" is simpler and match the VM name.
The precedent regex "^.*-name\x00([a-zA-Z0-9.-_-]*)\x00\-.*$" was not matching it.
BTW, It seems that \x00 does not match correctly null characters so I removed it.
