rpc: libssh: allow a NULL known_hosts file
rpc: libssh: allow a NULL known_hosts file

Make sure that virNetLibsshSessionSetHostKeyVerification accepts a NULL
value for the path to the known_hosts file:
- call ssh_options_set(SSH_OPTIONS_KNOWNHOSTS) anyway, using /dev/null,
  otherwise libssh will use its default path
- do not call ssh_write_knownhost when no known hosts file was set

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1406457

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