x86/viridian: fix xen-hvmcrash when vp_assist page is present

x86/viridian: fix xen-hvmcrash when vp_assist page is present

Currently use of xen-hvmcrash will cause an immediate domain_crash() in
initialize_vp_assist() because it is called from viridian_load_vcpu_ctxt()
without having first cleared any previous mapping.

This patch addes a check into viridian_load_vcpu_ctxt() to avoid re-
initialization and turned the domain_crash() in initialize_vp_assist()
into an ASSERT() since neither codepath into that function should allow
it to be hit.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>