qdev: Fix qdev_try_create() semantics

Since QOM'ification, qdev_try_create() uses object_new() internally,
which asserts "type != NULL" when the type is not registered.
This was revealed by the combination of kvmclock's kvm_enabled() check
and early QOM type registration.

Check whether the class exists before calling object_new(), so that
the caller (e.g., qdev_create) can fail gracefully, telling us which
device could not be created.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Cc: Anthony Liguori <aliguori@codemonkey.ws>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
