hostdev: report physfn

Each sr-iov virtual function is actually a separate function on a pci
bus. Libvirt reports these functions on the same level as physical
functions, but sysfs provides us means of determining which pf does
given vf belong to. This is useful in order to correctly construct the
tree.

Change-Id: Id1c41b1fd7e0cab51c869ae4a7882072be2c3fad
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35974
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>