libxl: Fix Xen 4.4 libxlVmStart logic

libxl: Fix Xen 4.4 libxlVmStart logic

ifdef LIBXL_HAVE_DOMAIN_CREATE_RESTORE_PARAMS hides a multi-line body
for a brace-less else.  Add braces to ensure proper logic is applied.

Without this fix, new domains cannot be started.  Both
libxl_domain_create_new and libxl_domain_create_restore are called when
starting a new domain leading to this error:
libxl: error: libxl.c:324:libxl__domain_rename: domain with name "guest" already exists.
libxl: error: libxl_create.c:800:initiate_domain_create: cannot make domain: -6