XEN 4.2 Issuing a shutdown before the ACPI power event is triggered puts the vm in a state where it ignores any other shutdown commands

[Diana Crisan <dcrisan@flexiant.com>]

Steps to replicate:
Using the config file below we create a vm with the xl create command.
Immediately as this returns we issue a shutdown using xl shutdown vm_id, which prints 
"PV control interface not available: external graceful shutdown not possible.
Use "-F" to fallback to ACPI power event.
shutdown failed (rc=-10)"
However, even after the vm boots up properly after this, it cannot be shutdown as it ignores any subsequent xl shutdown commands. We can only issue a kill after this point to destroy the vm.

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# The domain build function. HVM domain uses 'hvm'.
builder='hvm'

# Initial memory allocation (in megabytes) for the new domain.
#
# WARNING: Creating a domain with insufficient memory may cause out of
#          memory errors. The domain needs enough memory to boot kernel
#          and modules. Allocating less than 32MBs is not recommended.
memory = 512


# A name for your domain. All domains must have different names.
name = "UbuntuXen"

# 128-bit UUID for the domain.  The default behavior is to generate a new UUID
# on each call to 'xm create'.
#uuid = "06ed00fe-1162-4fc4-b5d8-11993ee4a8b9"

#-----------------------------------------------------------------------------
# The number of cpus guest platform has, default=1
vcpus=2


disk = [ 'tap:qcow2:/my/nfs/directory/testdisk.qcow2,xvda,w' ]

vif = ['mac=00:16:3e:25:96:c8 , bridge=defaultbr']

device_model_version = 'qemu-xen'
device_model_override = '/usr/lib/xen/bin/qemu-system-i386'
#device_model_override = '/usr/bin/qemu-system-x86_64'
#device_model_args = [ '-monitor', 'tcp:127.0.0.1:2345' ]

sdl=0

#----------------------------------------------------------------------------
# enable OpenGL for texture rendering inside the SDL window, default = 1
# valid only if sdl is enabled.
opengl=1

#----------------------------------------------------------------------------
# enable VNC library for graphics, default = 1
vnc=1

#----------------------------------------------------------------------------
# address that should be listened on for the VNC server if vnc is set.
# default is to use 'vnc-listen' setting from
# auxbin.xen_configdir() + /xend-config.sxp
vnclisten="0.0.0.0"

#----------------------------------------------------------------------------
# set VNC display number, default = domid
vncdisplay=0

#----------------------------------------------------------------------------
# try to find an unused port for the VNC server, default = 1
vncunused=0

#----------------------------------------------------------------------------
# set password for domain's VNC console
# default is depents on vncpasswd in xend-config.sxp
vncpasswd='password'


#----------------------------------------------------------------------------
# enable stdvga, default = 0 (use cirrus logic device model)
stdvga=0

#-----------------------------------------------------------------------------
#   serial port re-direct to pty deivce, /dev/pts/n
#   then xm console or minicom can connect
serial='pty'