qemu-char: do not leak QemuMutex when freeing a character device

The leak is only apparent on Win32.  On POSIX platforms destroying a
mutex is not necessary.

Reported-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
