pci-assign: do not test path with access() before opening

Using access() is a time-of-check/time-of-use race condition.  It is
okay to use them to provide better error messages, but that is pretty
much it.

In this case we can get the same error from fopen(), so just use
strerror and errno there---which actually improves the error
message most of the time.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
