pvpanic: introduce crashloaded for pvpanic

Add bit 1 for pvpanic. This bit means that guest hits a panic, but
guest wants to handle error by itself. Typical case: Linux guest runs
kdump in panic. It will help us to separate the abnormal reboot from
normal operation.
Signed-off-by: Nzhenwei pi <pizhenwei@bytedance.com>
Message-Id: <20200114023102.612548-2-pizhenwei@bytedance.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

docs/specs/pvpanic.txt

@@ -16,8 +16,12 @@ pvpanic exposes a single I/O port, by default 0x505. On read, the bits
 recognized by the device are set. Software should ignore bits it doesn't
 recognize. On write, the bits not recognized by the device are ignored.
 Software should set only bits both itself and the device recognize.
-Currently, only bit 0 is recognized, setting it indicates a guest panic
-has happened.
+
+Bit Definition
+--------------
+bit 0: setting it indicates a guest panic has happened.
+bit 1: named crashloaded. setting it indicates a guest panic and run
+       kexec to handle error by guest itself.
 
 ACPI Interface
 --------------