'make distcheck' has been broken since commit 21685c95; basically,
it emulates the case of a read-only $(srcdir) (such as building
from a tarball exploded onto a CD-ROM), but we were creating our
fake pci device as a symlink into $(srcdir) and failing when that
requires opening the config file for writing:

 3) testVirPCIDeviceReset                                             ... libvirt:  error : Failed to open config space file '/sys/bus/pci/devices/0000:00:01.0/config': Permission denied

Fix it by copying rather than symlinking.

* tests/virpcimock.c (make_file): Add parameter to allow binary
creation; adjust all callers.
(pci_device_new_from_stub): Copy rather than symlink.
Signed-off-by: NEric Blake <eblake@redhat.com>