[libvirt] [PATCH] dots should be valid characters in interface names

Howdy! I spent some time scratching my head this evening, as libvirt was stripping the target/@dev entry from my interface[@type='ethernet']. Turns out that this was caused by the interface name containing a period, which is rejected by isValidIfname(). As the kernel allows the creation of such interfaces, it is improper for libvirt to reject them. The attached one-liner (built against RHEL6b1's libvirt but still applicable against current master as of 10c681622) fixes this issue.