x86: Factor out vmx_handle_hypercall

Move hypercall handling from vmx_handle_exit into a separate function
for better code structuring. No functional changes.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
x86: Reject hypercalls issued by userspace contexts

Reject any hypercall issued by userspace contexts, thus enable cells to
establish proper access control to Jailhouse services.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>