The virsh-domain.c file was pretty self-contained; the only
entry point was the table of command definitions.  The bulk
of this patch is making more functions in virsh.c reusable.
A later patch will clean up poor naming choices.

* tools/Makefile.am (virsh_SOURCES): Build virsh-domain.c.
* tools/virsh-domain.h: New file.
* tools/virsh.h (virshReportError, vshResetLibvirtError)
(vshAskReedit, vshStreamSink): Declare.
* tools/virsh.c: Switch from using .c to .h.
(virshReportError, vshResetLibvirtError, vshAskReedit)
(vshStreamSink, prettyCapacity): Export.
(vshCatchInt): Move...
* tools/virsh-domain.c: ...into sole user.  Use header.