This is modelled after the iothread mutex lock. We keep a TLS flag to
indicate when that thread has acquired the lock and assert we don't
double-lock or release when we shouldn't have.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-Id: <20180227095237.1060.44661.stgit@pasha-VirtualBox>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>