This large, but mainly mechanical commit, renames "attach method"
everywhere to "backend".
Backwards compatibility of the API (guestfs_{set,get}_attach_method)
and environment (LIBGUESTFS_ATTACH_METHOD) is maintained, but in new
code use guestfs_{set,get}_backend and LIBGUESTFS_BACKEND instead.
The default backend (launching qemu directly) is now called 'direct'
instead of 'appliance', although you can still use 'appliance' as a
synonym.
68990840b6