CA-81449: Take VCPUs_at_startup into account when starting a guest.

Previously we were creating all the CPU devices as online.

n.b. adding the "online" option to Device.Vcpu.add means it's
functionally identical to Device.Vcpu.set

Signed-off-by: John Else <john.else@citrix.com>
Signed-off-by: David Scott <dave.scott@eu.citrix.com>