Having one .c file include another does not give any compilation
benefits; move towards modular .o files by first splitting out
reused declarations into a new virsh.h.  This patch doesn't try
very hard to see which functions are used or not, to make it
easier to review the file split.  Future patches can further trim
the header to be smaller.

* tools/Makefile.am (virsh_SOURCES): List new file, and prepare
for others.
* tools/virsh.c: Split declarations...
* tools/virsh.h: ...into new file, and make several functions
non-static.
* tools/virsh-domain-monitor.c (vshGetDomainDescription): Make
non-static.