virt-ls, virt-edit: Remove references to Sys::Guestfs::Lib and other irrelevant Perl libraries.
New API: feature-available.

This API is an easier to use version of the existing guestfs_available,
because the new API returns true/false instead of throwing an error
when a feature from the list is not available.

In truth we've had this implementation internally in the library
and several tools and in Sys::Guestfs::Lib for a long time.  This
change just turns it into a publicly consumable API.
tests/data: Update README and comments about initrd binary files.

These are used for testing the C, not Perl, file_architecture API.
perl: Remove use of Sys::Guestfs::Lib::open_guest function.

This obsolete function does all sorts of libvirt/XML things, which can
now be replaced by using ordinary API calls; especially $g->add_domain.
perl: Remove Sys::Guestfs::Lib.

See announcement on the mailing list for more details about this change:

https://www.redhat.com/archives/libguestfs/2013-April/msg00001.html
Version 1.21.26.
FAQ: In Fedora >= 19, the RPM detects if it can access the network when building.