gdbstub: let the debugger resume from guest panicked state

While in general we forbid a "continue" from the guest panicked
state, it makes sense to have an exception for that when continuing
in the debugger.  Perhaps the guest entered that state due to a bug,
for example, and we want to continue no matter what.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Message-id: 1370272015-9659-3-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
