util: allow \n to terminate password input

The qemu_read_password() method looks for \r to terminate the
reading of the a password. This is what will be seen when
reading the password from a TTY. When scripting though, it is
useful to be able to send the password via a pipe, in which
case we must look for \n to terminate password input.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
