Another file worth compiling on its own instead of by .c inclusion.

* tools/virsh-domain-monitor.h: New file.
* tools/Makefile.am (virsh_SOURCES): Build it.
* tools/virsh.h (vshGetDomainDescription): Move to correct
header.
* tools/virsh-domain-monitor.c: Use new header.
* tools/virsh.c: Likewise.
* tools/virsh-domain.c: Likewise.