libxl: preserve console tty across soft reset

libxl: preserve console tty across soft reset

On soft reset we remove the domain from xenstore and introduce it back to
have everything reconnected. Console, however, stays attached (as xenconsoled
checks if the domain is dying and our domain is not) but we lose the
information about tty:

before soft reset:
   console = ""
    ...
    type = "xenconsoled"
    output = "pty"
    tty = "/dev/pts/1"
    ...

after:
   console = ""
    ...
    type = "xenconsoled"
    output = "pty"
    tty = ""
    ...

The issue applies to both HVM and PVH but for HVM guests serial console
through QEMU is usually in use and for PVH we don't have it.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>