virsh: common code for waiting for an event

virsh: common code for waiting for an event

I plan to add 'virsh event' to virsh-domain.c and 'virsh
net-event' to virsh-network.c; but as they will share quite
a bit of common boilerplate, it's better to set that up now
in virsh.c.

* tools/virsh.h (_vshControl): Add fields.
(vshEventStart, vshEventWait, vshEventDone, vshEventCleanup): New
prototypes.
* tools/virsh.c (vshEventFd, vshEventOldAction, vshEventInt)
(vshEventTimeout): New helper variables and functions.
(vshEventStart, vshEventWait, vshEventDone, vshEventCleanup):
Implement new functions.
(vshInit, vshDeinit, main): Manage event timeout.

Signed-off-by: Eric Blake <eblake@redhat.com>