vl: allow "cont" from panicked state

After reporting the GUEST_PANICKED monitor event, QEMU stops the VM.
The reason for this is that events are edge-triggered, and can be lost if
management dies at the wrong time.  Stopping a panicked VM lets management
know of a panic even if it has crashed; management can learn about the
panic when it restarts and queries running QEMU processes.  The downside
is of course that the VM will be paused while management is not running,
but that is acceptable if it only happens with explicit "-device pvpanic".

Upon learning of a panic, management (if configured to do so) can pick a
variety of behaviors: leave the VM paused, reset it, destroy it.  In
addition to all of these behaviors, it is possible to dump the VM core
from the host.

However, right now, the panicked state is irreversible, and can only be
exited by resetting the machine.  This means that any policy decision
is entirely in the hands of the host.  In particular there is no way to
use the "reboot on panic" option together with pvpanic.

This patch makes the panicked state reversible (and removes various
workarounds that were there because of the state being irreversible).
With this change, management has a wider set of possible policies: it
can just log the crash and leave policy to the guest, it can leave the
VM paused.  In particular, the "log the crash and continue" is implemented
simply by sending a "cont" as soon as management learns about the panic.
Management could also implement the "irreversible paused state" itself.
And again, all such actions can be coupled with dumping the VM core.

Unfortunately we cannot change the behavior of 1.6.0.  Thus, even if
it uses "-device pvpanic", management should check for "cont" failures.
If "cont" fails, management can then log that the VM remained paused
and urge the administrator to update QEMU.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
