virtual: Move mutex into p11_library_init()

virtual: Move mutex into p11_library_init()

We used to provide p11_virtual_fixed_{,un}init() to only initialize a
mutex used in virtual.c.  That required all the tests calling virtual
functions to call p11_virtual_fixed_{,un}init() in main().

For simplicity, move the mutex variable initialization into
p11_library_init().