tools/libxc: check to set args.mmio_size before call xc_hvm_build

tools/libxc: check to set args.mmio_size before call xc_hvm_build

After commit 5dff8e9eedc7, "libxc/libxl: fill xc_hvm_build_args in
libxl" is introduced, we won't check to set args.mmio_size inside
xc_hvm_build as before. So instead, we need to do this before call
that.

CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>