Calling error_report() from within a function that takes an Error **
argument is suspicious.  drive_new() calls error_report() even though
it can run within drive_init_func(), which takes an Error ** argument.
drive_init_func()'s caller main(), via qemu_opts_foreach(), is fine
with it, but clean it up anyway:

* Convert drive_new() to Error

* Update add_init_drive() to report the error received from
  drive_new()

* Make main() pass &error_fatal through qemu_opts_foreach(),
  drive_init_func() to drive_new()

* Make default_drive() pass &error_abort through qemu_opts_foreach(),
  drive_init_func() to drive_new()

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20181017082702.5581-34-armbru@redhat.com>