When listening for a subset of monitor events, it can be tedious
to register for each event name in series; nicer is to register
for multiple events in one go.  Implement a flag to use regex
interpretation of the event filter.

While at it, prove how much I hate the shift key, by adding a
way to filter for 'shutdown' instead of 'SHUTDOWN'. :)

* include/libvirt/libvirt-qemu.h
(virConnectDomainQemuMonitorEventRegisterFlags): New enum.
* src/libvirt-qemu.c (virConnectDomainQemuMonitorEventRegister):
Document flags.
* tools/virsh-domain.c (cmdQemuMonitorEvent): Expose them.
* tools/virsh.pod (qemu-monitor-event): Document this.
* src/conf/domain_event.c
(virDomainQemuMonitorEventStateRegisterID): Add flags.
(virDomainQemuMonitorEventFilter): Handle regex, and optimize
client side.
(virDomainQemuMonitorEventCleanup): Clean up regex.
Signed-off-by: NEric Blake <eblake@redhat.com>