parallels: substitute parallels with vz in strings

Here we stop referencing vz driver by different names
in error messages. 'parallels driver', 'Parallels Cloud
Server', 'Parallels driver' all become just 'vz driver'.
No functional changes. Only renaming and a bit of rewording.
Signed-off-by: NMaxim Nestratov <mnestratov@parallels.com>