hw/pci-assign: split pci-assign.c

We will try to reuse assign_dev_load_option_rom in xen side, and
especially its a good beginning to unify pci assign codes both on
kvm and xen in the future.

[Fix build for Windows]

Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>