vl: exit qemu on guest panic if -no-shutdown is not set

For automated testing purposes it can be helpful to exit qemu
(poweroff) when the guest panics. Make this the default unless
-no-shutdown is specified.

For internal-errors like errors from KVM_RUN the behaviour is
not changed, in other words QEMU does not exit to allow debugging
in the QEMU monitor.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <1476775794-108012-1-git-send-email-borntraeger@de.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>