pci: don't hw_error() when no slot is available.

pci: don't hw_error() when no slot is available.

Current PCI code will simply hw_error() and thus abort in case no free
PCI slot is available or the requested PCI slot is already in use by
another device.  For the hotplug case this behavior is not acceptable.
This patch makes qemu pass up the error properly, so the calling code
can decide whenever it wants to exit with an error (on startup) or
whenever it wants to continue (hotplug).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 09e3acc6cfabfd85a9dacc04471df5f05019c779)