xenguest-4.2: memset the xc_hvm_build_args structure

We're not loading any SMBIOS or ACPI modules but we are suffering

  "Insufficient space to load modules"

errors while building the domain. This could happen if the SMBIOS
or ACPI module lengths in the structure contain junk, such that
their sum > mem_size of the domain (in MiB)

If we're lucky this will fix [xapi-project/xenopsd#69]

Signed-off-by: David Scott <dave.scott@citrix.com>