exec: Atomic access to bounce buffer

There could be a race condition when two processes call
address_space_map concurrently and both want to use the bounce buffer.

Add an in_use flag in BounceBuffer to sync it.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1426496617-10702-2-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
