virnetsocket: improve search for default SSH key
virnetsocket: improve search for default SSH key

Add a couple of helper functions to check whether one of the default
names of SSH keys (as documented in ssh-keygen(1)) exists, and use them
to specify a key for the libssh2 transport if none was passed.

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