qemu: Report less errors on driver startup
qemu: Report less errors on driver startup

It is not a problem at all if the `tss` user/group does not exist, the code
fallbacks to the `root` user/group.  However we report a warning for no reason
on every start-up.  Fix this by checking if the user/group actually exists.

Signed-off-by: Martin Kletzander <e82f23e74d43760279ebcc505dac4ae70d5627d0@redhat.com>

eskultety/libvirt,jfehlig/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,libvirt/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,andreabolognani/libvirt,zippy2/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,eskultety/libvirt,eskultety/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,nertpinx/libvirt,olafhering/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,crobinso/libvirt,zippy2/libvirt,eskultety/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jfehlig/libvirt,crobinso/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,eskultety/libvirt,nertpinx/libvirt,crobinso/libvirt