gdbstub: Catch and report more vmstop reasons

gdbstub: Catch and report more vmstop reasons

When the VM goes into stop state while there is a gdb frontend attached,
it makes sense to inform gdb about this fact and at least a bit about
the stop reason. Basically, all stops are interesting except for the
temporary VMSTOP_SAVE/LOADVM.

The patch maps the relevant VMSTOP reasons on unique and more or less
associatable signals that gdb understands.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>