libxl: fix libxl_string_list_length and its only caller

libxl: fix libxl_string_list_length and its only caller

The wrong amount of indirections were being taken in
libxl_string_list_length, and its only caller was miscounting the amount
of initial non-list arguments, seemingly since the initial commit
(599c784).

This has been seen and reported in the wild (##xen):
< Trixboxer> Hi, any idea why would I get
< Trixboxer> xl: libxl_bootloader.c:42: bootloader_arg: Assertion `bl->nargs < bl->argsspace' failed.
< Trixboxer> 4.2.2-23.el6

Coverity-ID: 1054954
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>