kvm: Switch to unlocked MMIO

Do not take the BQL before dispatching MMIO requests of KVM VCPUs.
Instead, address_space_rw will do it if necessary. This enables completely
BQL-free MMIO handling in KVM mode for upcoming devices with fine-grained
locking.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1434646046-27150-10-git-send-email-pbonzini@redhat.com>