Convert all top-level nested VM-Enter consistency check functions to
return 0/-EINVAL instead of failure codes, since now they can only
ever return one failure code.

This also does not give the false impression that failure information is
always consumed and/or relevant, e.g. vmx_set_nested_state() only
cares whether or not the checks were successful.

nested_check_host_control_regs() can also now be inlined into its caller,
nested_vmx_check_host_state, since the two have effectively become the
same function.

Based on a patch by Sean Christopherson.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>