xen-pvdevice: Introduce a simplistic xen-pvdevice save state

xen-pvdevice: Introduce a simplistic xen-pvdevice save state

This should help to avoid problems with accessing the device after
migration/resume without PV drivers by migrating its PCI configuration
space state. Without an explicitly defined state record it resets
every time a VM migrates which confuses the OS and makes every
access to xen-pvdevice MMIO region to fail. PV tools enable some
logic to save and restore PCI configuration state from within the VM
every time it migrates which basically hides the issue.

Older systems will acquire the new record when migrated which should
not change their state for worse.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>