Introduce new mutex locking functions mutex_trylock_nest_lock() and
mutex_lock_killable_nest_lock() and use them to clean up locking
of all vCPUs for a VM.
For x86, this removes some complex code that was used instead
of lockdep's "nest_lock" feature.
For ARM and RISC-V, this removes a lockdep warning when the VM is
configured to have more than MAX_LOCK_DEPTH vCPUs, and removes a fair
amount of duplicate code by sharing the logic across all architectures.
Signed-off-by: Paolo BOnzini <pbonzini@redhat.com>