libxl: Use LOG_ERRNO rather than ERRNOVAL in libxl_pci.c

The xc_* functions (now) return -1 on error and set errno.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>