bql: add a "mock" BQL for Rust unit tests

bql: add a "mock" BQL for Rust unit tests

Right now, the stub BQL in stubs/iothread-lock.c always reports itself as
unlocked.  However, Rust would like to run its tests in an environment where
the BQL *is* locked.  Provide an extremely dirty function that flips the
return value of bql_is_locked() to true.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>