libxl: HVM firmware passthrough support

libxl: HVM firmware passthrough support

This patch introduces support for two new parameters in libxl:

smbios_firmware=<path_to_smbios_structures_file>
acpi_firmware=<path_to_acpi_tables_file>

The changes are primarily in the domain building code where the firmware files
are read and passed to libxc for loading into the new guest. After the domain
building call to libxc, the addresses for the loaded blobs are returned and
written to xenstore.

LIBXL_HAVE_FIRMWARE_PASSTHROUGH is defined in libxl.h to allow users to
determine if the feature is present.

This patch also updates the xl.cfg man page with descriptions of the two new
parameters for firmware passthrough.

Signed-off-by: Ross Philipson <ross.philipson@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>