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_unplug() command and introduces it
in the disk removal paths.  There is some discussion to be had about how
to handle the case where the guest is running in a QEMU without this
command (and the fact that we currently don't have a way of detecting
what monitor commands are available).

Changes since v2:
 - use VIR_ERROR to report when unplug command not found
Changes since v1:
 - return > 0 when command isn't present, < 0 on command failure
 - detect when drive_unplug command isn't present and log error
   instead of failing entire command
Signed-off-by: NRyan Harper <ryanh@us.ibm.com>