Add version numbers to Perl modules (RHBZ#521674).

Sys::Guestfs now contains a version number which reflects the
ABI that we are compiled against, ie. '0.<MAX_PROC_NR>'.  This has
the beneficial side effect of causing an error if the user tries
to mix versions of the Perl module and the XS code.

Sys::Guestfs::Lib now contains a synthetic version number which
will reflect future changes in that module.