Fix pci_add storage not to exit on bad first argument

Monitor command "pci_add ADDR storage ..." does its work in
qemu_pci_hot_add_nic().  It called pci_create(..., ADDR) to create the
device.  That's wrong, because pci_create() terminates the program
when ADDR is invalid.

Use pci_get_bus_devfn() and pci_create_noinit() instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
