qga-win: Enable 'can-offline' field in 'guest-get-vcpus' reply

The QGA schema states:

@can-offline: Whether offlining the VCPU is possible. This member
               is always filled in by the guest agent when the structure
               is returned, and always ignored on input (hence it can be
               omitted then).

Currently 'can-offline' is missing entirely from the reply. This causes
errors in libvirt which is expecting the reply to be compliant with the
schema docs.

BZ#1438735: https://bugzilla.redhat.com/show_bug.cgi?id=1438735

Signed-off-by: Sameeh Jubran <sameeh@daynix.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
