rbd: Fix use after free in qemu_rbd_set_keypairs() error path
    
    If we want to include the invalid option name in the error message, we
    can't free the string earlier than that.
    
    Cc: qemu-stable@nongnu.org
    Signed-off-by: Kevin Wolf <kwolf@redhat.com>
    Reviewed-by: Max Reitz <mreitz@redhat.com>
    Reviewed-by: Eric Blake <eblake@redhat.com>
    (cherry picked from commit 71c87815f9e0386b6f3e22942adc956fd603c82f)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>