i386: only parse the initrd_filename once for multiboot modules

i386: only parse the initrd_filename once for multiboot modules

The multiboot code parses the initrd_filename twice, first to count how
many entries there are, and second to process each entry. This changes
the first loop to store the parse module names in a list, and the second
loop can now use these names. This avoids having to pass NULL to the
get_opt_value() method which means it can safely assume a non-NULL param.

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]>