pci: refuse empty ROM files

A zero size ROM file is invalid and should produce a warning.
Attempting to use a zero size file ends up hitting an assertion
qemu_ram_set_idstr() because RAMBlocks with duplicate addresses are
allocated - due to zero size the allocator doesn't increment the next
available RAMBlock offset.

Also convert __FUNCTION__ to __func__ while we're touching this code.
There are no other __FUNCTION__ instances in pci.c anymore.

Reported-by: Milos Ivanovic <milosivanovic@orcon.net.nz>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
