UserConfig: virify: check username vs userid

UserConfig: virify: check username vs userid

Since we have both userid and username in the schema and
both have a minimum length of 4, creating a user named 'foo'
was previously possible because the 'username' property was
not checked. Loading the file back in then failed because at
load time, the username 'foo' was too short.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>