tools: Introduce new client generic module vsh

tools: Introduce new client generic module vsh

In order to share as much virsh' logic as possible with upcomming
virt-admin client we need to split virsh logic into virsh specific and
client generic features.

Since majority of virsh methods should be generic enough to be used by
other clients, it's much easier to rename virsh specific data to virshX
than doing this vice versa. It moved generic virsh commands (including info
and opts structures) to generic module vsh.c.

Besides renaming methods and structures, this patch also involves introduction
of a client specific control structure being referenced as private data in the
original control structure, introduction of a new global vsh Initializer,
which currently doesn't do much, but there is a potential for added
functionality in the future.
Lastly it introduced client hooks which are especially necessary during
client connecting phase.