hw/ipmi: Fix realize() error API violations

isa_ipmi_bt_realize(), ipmi_isa_realize(), pci_ipmi_bt_realize(), and
pci_ipmi_kcs_realize() dereference @errp when IPMIInterfaceClass
method init() fails.  That's wrong; see the big comment in error.h.
Introduced in commit 0719029c "ipmi: Add an ISA KCS low-level
interface", then imitated in commit a9b74079 "ipmi: Add a BT
low-level interface" and commit 12f983c6 "ipmi: Add PCI IPMI
interfaces".

No caller actually passes null.

Fix anyway: splice in a local Error *err, and error_propagate().

Cc: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191204093625.14836-9-armbru@redhat.com>