multiboot: copy the cmdline verbatim, unescape module strings

get_opt_value() truncates the value at the first comma
Use memcpy() instead so that -append works correctly in the
presence of commas.  For -initrd to work right, instead,
unescape the module filename and parameters with get_opt_value()
before calling mb_add_cmdline().

Signed-off-by: Vlad Lungu <vlad.lungu@windriver.com>
Message-Id: <1481805124-16242-1-git-send-email-vlad.lungu@windriver.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>