ioemu: handle empty vnc passwd

ioemu: handle empty vnc passwd
Have xenstore_read_vncpasswd return -1 when it is unable to read the
passwd from XenStore (and store an empty password).  However, don't
exit in such case since it may just mean that the use didn't set a
passwd. If he really did, xend would have given the passwd flag in the
-vnc option, and the empty passwd would make the vnc authentication
reject any password anyway.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>
Acked-by: Daniel P. Berrange <berrange@redhat.com>