Introduce per-hypervisor virtual RPMs

Introduce per-hypervisor virtual RPMs

Introduce a set sub-RPMs, one per hypervisor, which can be used
as dependency targets by applications wishing to pull in the
full stack of packages required for a specific hypervisor. This
avoids the application needing to know what the hypervisor specific
package set is.

ie, applications should not need to know that using the libvirt
Xen hypervisor requires the 'xen' RPM - libvirt should take care
of that knowledge. All the application wants is 'libvirt-daemon-xen'

There are 5 sub-RPMs:

  libvirt-daemon-qemu - non-native TCG based emulators
  libvirt-daemon-kvm  - native KVM hypervisor
  libvirt-daemon-uml  - User Mode linux
  libvirt-daemon-xen  - Xen, either via XenD or libxl
  libvirt-daemon-lxc  - Linux native containers

When driver modules get turned on, these sub-RPMs will also
gain dependencies on the appropriate driver module .so files