libqos: split I2CAdapter initialization and allocation

Provide *_init functions that populate an I2CAdapter struct without
allocating one, and make the existing *_create functions wrap them.

Because in the new setup *_create might return a pointer inside the
IMXI2C or OMAPI2C struct, create companion *_free functions to go
back to the outer pointer.

All this is temporary until allocation will be handled entirely by
qgraph.

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