Document possible hypervisor bug in serial_console

This is described in Issue 83.

Update both the install guide and the gnt-instance manpage.
In the install guide move another KVM specific example inside a new KVM
admonition for the hypervisor parameters section.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>