SysBus EHCI was introduced in a hurry before 1.3 Soft Freeze.
To use QOM casts in place of DO_UPCAST() / FROM_SYSBUS(), we need an
identifying type. Introduce generic abstract base types for PCI and
SysBus EHCI to allow multiple types to access the shared fields.

While at it, move the state structs being amended with macros to the
header file so that they can be embedded.

The VMSTATE_PCI_DEVICE() macro does not play nice with the QOM
parent_obj naming convention, so defer that cleanup.
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>