Commit 4ae59411 introduced the ability to make probing for
device properties conditional on a capability being set, but
didn't extend the use of this feature to existing devices.

This commit does the last bit of work, which results in a lot
of pointless QMP chatter no longer happening and our test suite
shrinking a fair bit.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>