fix CPUID vendor override

the meaning of vendor_override is actually the opposite of how it
is currently used :-(
Fix it to allow KVM to export the non-native CPUID vendor if
explicitly requested by the user.

The intended behavior is:
With TCG:
  - always inject the configured vendor (either hard-coded, in config
    files or via ",vendor=" commandline)
With KVM:
  - by default inject the host's vendor
  - if the user specifies ",vendor=" on the commandline, use this
    instead of the host's vendor
  - all pre-configured vendors (hard-coded, config file) are ignored

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
