This patch enables modifying network device configuration using the
virUpdateDeviceFlags API method. Matching of devices is accomplished
using MAC addresses.

While updating live configuration of a running domain, the user is
allowed only to change link state of the interface. Additional
modifications may be added later. For now the code checks for
unsupported changes and thereafter changes the link state, if
applicable.

When updating persistent configuration of guest's network interface the
whole configuration (except for the MAC address) may be modified and
is stored for the next startup.

* src/qemu/qemu_driver.c   - Add dispatching of virUpdateDevice for
                             network devices update (live/config)
* src/qemu/qemu_hotplug.c  - add setting of initial link state on live
                             device addition
                           - add function to change network device
                             configuration. By now it supports only
                             changing of link state
* src/qemu/qemu_hotplug.h  - Headers to above functions
* src/qemu/qemu_process.c  - set link states before virtual machine
                             start. Qemu does not support setting of
                             this on the command line.