qemu: refactor qemuDomainCheckDiskPresence for only disk presence check
qemu: refactor qemuDomainCheckDiskPresence for only disk presence check

Refactor this function to make it focus on disk presence checking,
including diskchain checking, and not only for CDROM and Floppy.
This change is good for the following patches.

taget/libvirt,andreabolognani/libvirt,shugaoye/libvirt,zippy2/libvirt,eskultety/libvirt,nertpinx/libvirt,andreabolognani/libvirt,datto/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,agx/libvirt,rlaager/libvirt,olafhering/libvirt,agx/libvirt,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,cbosdo/libvirt,datto/libvirt,cbosdo/libvirt,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,cbosdo/libvirt,olafhering/libvirt,VenkatDatta/libvirt,libvirt/libvirt,VenkatDatta/libvirt,taget/libvirt,jfehlig/libvirt,elmarco/libvirt,elmarco/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,eskultety/libvirt,cbosdo/libvirt,elmarco/libvirt,datto/libvirt,andreabolognani/libvirt,eskultety/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,agx/libvirt,elmarco/libvirt,eskultety/libvirt,rlaager/libvirt,shugaoye/libvirt,olafhering/libvirt,shugaoye/libvirt,rlaager/libvirt,rlaager/libvirt,rlaager/libvirt,crobinso/libvirt,jardasgit/libvirt,jardasgit/libvirt,taget/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,andreabolognani/libvirt,shugaoye/libvirt,eskultety/libvirt,agx/libvirt,elmarco/libvirt,crobinso/libvirt,agx/libvirt,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,crobinso/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,taget/libvirt,cbosdo/libvirt,jardasgit/libvirt,nertpinx/libvirt,datto/libvirt,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,jardasgit/libvirt,libvirt/libvirt,libvirt/libvirt,libvirt/libvirt,taget/libvirt