libxl: Allow multiple serial ports on HVM domain creation

libxl: Allow multiple serial ports on HVM domain creation

This patch allows an HVM domain to be created with multiple serial
ports.

Since the previous interface only allowed the passing of a single
device, this requires us to add a new element to the hvm struct of
libxl_domain_build_info -- serial_list.  For API compatibility, the
old element, serial, remains.

If hvm.serial_list is set, each device listed will cause an extra
"-serial [foo]" to be appended to the qemu command line.

Callers may set either hvm.serial or hvm.serial_list, but not
both; libxl will throw an error if both are set.

In order to allow users of libxl to write software compatible with
older versions of libxl, also define LIBXL_HAVE_BUILDINFO_SERIAL_LIST.
If this is defined, callers may use either hvm.serial or
hvm.serial_list; otherwise, only hvm.serial will be available.

This patch borrows substantially from the multiple USB device patch
ac16730d0339d41fd7d129a5cb2d40ed67a303d9.

Signed-off-by: Ed White <edmund.h.white@intel.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>