qemu: pass iscsi authorization credentials

qemu: pass iscsi authorization credentials

A better way to do this would be to use a configuration file like

   [iscsi "target-name"]
   user = name
   password = pwd

and pass it via -readconfig.  This would remove the username and password
from the "ps" output.  For now, however, keep this solution.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>