xenconfig: check return value of regcomp
xenconfig: check return value of regcomp

Commit ec63000a missed checking the return value of regcomp(),
which coverity promptly identified.

crobinso/libvirt,olafhering/libvirt,VenkatDatta/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,rlaager/libvirt,datto/libvirt,zippy2/libvirt,jfehlig/libvirt,andreabolognani/libvirt,taget/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,rlaager/libvirt,libvirt/libvirt,datto/libvirt,jfehlig/libvirt,libvirt/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,olafhering/libvirt,taget/libvirt,jfehlig/libvirt,crobinso/libvirt,fabianfreyer/libvirt,zippy2/libvirt,rlaager/libvirt,eskultety/libvirt,fabianfreyer/libvirt,taget/libvirt,rlaager/libvirt,jfehlig/libvirt,andreabolognani/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,libvirt/libvirt,olafhering/libvirt,jardasgit/libvirt,datto/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,libvirt/libvirt,taget/libvirt,zippy2/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,nertpinx/libvirt,eskultety/libvirt,datto/libvirt,datto/libvirt,eskultety/libvirt,rlaager/libvirt,crobinso/libvirt,taget/libvirt,andreabolognani/libvirt,eskultety/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,eskultety/libvirt