KVM: Use a big lock to replace per-kml slots_lock

Per-kml slots_lock will bring some trouble if we want to take all slots_lock of
all the KMLs, especially when we're in a context that we could have taken some
of the KML slots_lock, then we even need to figure out what we've taken and
what we need to take.

Make this simple by merging all KML slots_lock into a single slots lock.

Per-kml slots_lock isn't anything that helpful anyway - so far only x86 has two
address spaces (so, two slots_locks).  All the rest archs will be having one
address space always, which means there's actually one slots_lock so it will be
the same as before.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210506160549.130416-3-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>