cutils: Fix qemu_strtosz() & friends to reject non-finite sizes

qemu_strtosz() & friends reject NaNs, but happily accept infinities.
They shouldn't. Fix that.

The fix makes use of qemu_strtod_finite(). To avoid ugly casts,
change the @end parameter of qemu_strtosz() & friends from char **
to const char **.

Also, add two test cases, testing that "inf" and "NaN" are properly
rejected. While at it, also fixup the function documentation.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20181121164421.20780-3-david@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>