Currently libvirt doesn't confirm whether the guest has responded to the
disk removal request.  In some cases this can leave the guest with
continued access to the device while the mgmt layer believes that it has
been removed.  With a recent qemu monitor command[1] we can
deterministically revoke a guests access to the disk (on the QEMU side)
to ensure no futher access is permitted.

This patch adds support for the drive_del() command and introduces it
in the disk removal paths.  If the guest is running in a QEMU without this
command we currently explicitly check for unknown command/CommandNotFound
and log the issue.

If QEMU supports the command we issue the drive_del command after we attempt
to remove the device.  The guest may respond and remove the block device
before we get to attempt to call drive_del.  In that case, we explicitly check
for 'Device not found' from the monitor indicating that the target drive
was auto-deleted upon guest responds to the device removal notification.

1. http://thread.gmane.org/gmane.comp.emulators.qemu/84745Signed-off-by: NRyan Harper <ryanh@us.ibm.com>