engine: Fixes in non plugin console invocation

This patch introduces minor fixes in non plugin console invocation mainly
related to internet explorer.

Changes in backend:
 - mapping of attachment servlet changed to "console.vv", since IE ignores
   Attachment header
 - the filename parameter was removed as it has no effect in IE
 - no-cache header changed to max-age=0 header. With no-cache header IE fails
   to download the file if https is used
 - assume UTF-8 as the encoding of the incoming request in the attachment
   servlet

Changes in frontend:
 - added spice plugin detection for IE

Change-Id: I7fe6fbf274a9ac215e8f6593cc000f778a431928
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>