i386: fix regression parsing multiboot initrd modules

i386: fix regression parsing multiboot initrd modules

The logic for parsing the multiboot initrd modules was messed up in

  commit 950c4e6c94b15cd0d8b63891dddd7a8dbf458e6a
  Author: Daniel P. Berrangé <[email protected]>
  Date:   Mon Apr 16 12:17:43 2018 +0100

    opts: don't silently truncate long option values

Causing the length to be undercounter, and the number of modules over
counted. It also passes NULL to get_opt_value() which was not robust
at accepting a NULL value.

Signed-off-by: Daniel P. Berrangé <[email protected]>
Message-Id: <20180514171913[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Tested-by: Roman Kagan <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>