libxl: Introduce functions to add and remove USB devices to an HVM guest

This patch exposes a generic interface which can be expanded in the
future to implement USB for PVUSB, qemu, and stubdoms.  It can also be
extended to include other types of USB other than host USB (for example,
tablets, mice, or keyboards).

For each device removed or added, one of two protocols is available:
* PVUSB
* qemu (DEVICEMODEL)

The caller can additionally specify "AUTO", in which case the library will
try to determine the best protocol automatically.

At the moment, the only protocol implemented is DEVICEMODEL, and the only
device type impelmented is HOSTDEV.

This uses the qmp functionality, and is thus only available for
qemu-xen, not qemu-traditional.

History:
v7:
 - Refresh
v6:
 - Return a proper error code if libxl__xs_mkdir fails
 - Make casts cuddly
 - Add a space after switch statements
v5:
 - fix erroneous use of NOGC in qmp functions
 - Don't check return of libxl__sprintf in libxl_create.c
 - Check return values of new xs actions in libxl_create.c
 - Use updated template for xenstore transactions, do checks
 - use xs_read_checked
 - Fixed if (x) spacing to match coding style
 - use GCSFPRINTF
 - use libxl__calloc
 - Fix comment for LIBXL_HAVE_USB
 - use idl-generated *_from_string() and *_to_string() functions

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
CC: Ian Jackson <ian.jackson@citrix.com>
CC: Roger Pau Monne <roger.pau@citrix.com>
CC: sstanisi@cbnco.com
xl: Add commands for usb hot-plug

v7:
 - Refresh
v6:
 - Rename to usb-attach and usb-detach to fit better w/ other device
   add/remove commands
 - Use positional arguments, to fit better w/ other device add/remove
   commands
 - Change specifier to "hostdev:xx.yy", looking forward to a day when
   we may be able to accept something like "tablet"
 - Style fix-ups
v5:
 - Remove extraneous is_hex
 - Replace is_dec with ctype.h isdigit()
 - Replace domid -1 with INVALID_DOMID
 - Fix up a couple of mistaken function names in strings
 - Copy CTYPE macro from libxl_internal.h
 - Fix to usb-list manpage (use -d domid)

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
CC: Ian Jackson <ian.jackson@citrix.com>
CC: Roger Pau Monne <roger.pau@citrix.com>
CC: sstanisi@cbnco.com