libxc: pass arguments to xc_hvm_build() in a structure

libxc: pass arguments to xc_hvm_build() in a structure

To allow new parameters to be added to the xc_hvm_build*() family of
functions, pass them in a structure.  Make the other variants fill in
the structure and call xc_hvm_build() (except for xc_hvm_build_mem()
which had no users and is removed).

The units of the mem_size and mem_target arguments are in bytes (not
MiB like the old functions).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>