libvirt/qemu - clean up UpdateDevice for consolidation.

libvirt/qemu - clean up UpdateDevice for consolidation.

This patch strips reusable part of qemuDomainUpdateDeviceFlags()
and consolidate it to qemuDomainModifyDeviceFlags().
No functional changes.

* src/qemu/qemu_driver.c
(qemuDomainChangeDiskMediaLive) : pulled out code for updating disks.
(qemuDomainUpdateDeviceLive) : core of UpdateDevice, extracted from
UpdateDeviceFlags()
(qemuDomainModifyDeviceFlags): add support for updating device in live domain.
(qemuDomainUpdateDeviceFlags): reworked as a wrapper function of
qemuDomainModifyDeviceFlags()

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Eric Blake <eblake@redhat.com>