qemuAgentGetUsers: Fix return value on success
qemuAgentGetUsers: Fix return value on success

Return 0 on success to match the documentation. The callers only check
for negative values.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

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