launch: Store drive information in guestfs_h

launch: Store drive information in guestfs_h

This is a NFC on its own, but provides a place-holder for drive metadata which
can be used after launch.

Fixes by RWMJ:

 - Fix the tests: this requires a new internal function 'debug-drives'
   that dumps out the g->drives information so it can be checked in
   two of the tests.  Previously these tests used 'debug-cmdline'.

 - Test file existence / use_cache_off in the add_drive_opts function,
   not when launching qemu in the child process.

 - Call free along error paths.

 - Add comments.