kvm: nVMX: Reorder error checks for emulated VMXON

Checks on the operand to VMXON are performed after the check for
legacy mode operation and the #GP checks, according to the pseudo-code
in Intel's SDM.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>