accel/tcg: Fix monitor deadlock
Commit 7bed89958bfb ("device_core: use drain_call_rcu in in qmp_device_add")
introduced a regression in QEMU 6.0 : passing device_add without argument
hangs the monitor. This was reported against qemu-system-mips64 with TGC,
but I could consistently reproduce it with other targets (x86 and ppc64).
See https://gitlab.com/qemu-project/qemu/-/issues/650 for details.
The problem is that an emulated busy-looping vCPU can stay forever in
its RCU read-side critical section and prevent drain_call_rcu() to return.
This series fixes the issue by letting RCU kick vCPUs out of the read-side
critical section when drain_call_rcu() is in progress. This is achieved
through notifiers, as suggested by Paolo Bonzini.