qemu-pr-helper: Actually allow users to specify pidfile
    
    Due to wrong specification of arguments to getopt_long() any
    attempt to set pidfile resulted in:
    
    1) the default to be leaked
    2) the @pidfile variable to be set to NULL (because optarg is
    NULL without this patch).
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Message-Id: <6f10cd53d361a395aa0e85a9311ec4e9a8fc11e5.1521868451.git.mprivozn@redhat.com>
    Cc: qemu-stable@nongnu.org
    Reviewed-by: Eric Blake <eblake@redhat.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    (cherry picked from commit f8e1a989644f22ba2f7afb0e13b6ce2309ea9503)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>