qom: allow QOM to be linked into tools binaries

qom: allow QOM to be linked into tools binaries

The qom objects are currently added to common-obj-y
which is only linked into the system emulators. The
later crypto patches will depend on QOM infrastructure
and will also be used from tools binaries. Thus the QOM
objects are moved into a new qom-obj-y variable which
can be referenced when linking tools, system emulators
and tests.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>