Show compiled in options in virsh -V

Show compiled in options in virsh -V

  To ease debugging this trivial patch allows to find what was compiled
in in the local version of libvirt, this doesn't work for remote access
but that's probably sufficient. With the patch I get on my machine:

paphio:~/libvirt/tools -> ./virsh -V
Virsh command line tool of libvirt 0.8.4
See web site at http://libvirt.org/

Compiled with support for:
 Hypervisors: Xen QEmu/KVM UML OpenVZ LXC ESX PHYP Test
 Networking: Remote Daemon Network Bridging Netcf Nwfilter
 Storage: Dir Disk Filesystem SCSI Multipath iSCSI LVM
 Miscellaneous: SELinux Secrets Debug Readline
paphio:~/libvirt/tools ->

* tools/virsh.c: add -V option
* tools/virsh.pod: document the extension