blockdev: Clean up abuse of DriveBackup member format

blockdev: Clean up abuse of DriveBackup member format

drive-backup argument @format defaults to the format of the source
unless @mode is "existing".

drive_backup_prepare() implements this by copying the source's
@format_name to DriveBackup member @format.  It leaves @has_format
false, violating the "has_format == !!format" invariant.  Unclean.
Falls apart when we elide @has_format (commit after next): then QAPI
passes @format, which is a string constant, to g_free().  iotest 056
duly explodes.

Clean it up.  Since the value stored in member @format is not actually
used outside this function, use a local variable instead of modifying
the QAPI object.

Signed-off-by: Markus Armbruster <[email protected]>
Cc: Kevin Wolf <[email protected]>
Cc: Hanna Reitz <[email protected]>
Cc: [email protected]
Message-Id: <20221104160712.3005652[email protected]>
Reviewed-by: Eric Blake <[email protected]>