fix vm_resume and allow vm_start with QMP status 'shutdown'

fix vm_resume and allow vm_start with QMP status 'shutdown'

When the VM is in status 'shutdown', i.e. after the guest issues a
powerdown while a backup is running, QEMU requires a 'system_reset' to
be issued before 'cont' can boot the guest again.

Additionally, when the VM has been powered down during a backup, the
logically correct call would be a 'vm_start', so automatically vm_resume
from vm_start in case this situation occurs. This also means the GUI can
cope with this almost unchanged.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>