KVM: VMX: Fold ept_update_paging_mode_cr4() into its caller

ept_update_paging_mode_cr4() accesses vcpu->arch.cr4 directly, which usually
needs to be accessed via kvm_read_cr4().  In this case, we can't, since cr4
is in the process of being updated.  Instead of adding inane comments, fold
the function into its caller (vmx_set_cr4), so it can use the not-yet-committed
cr4 directly.

Signed-off-by: Avi Kivity <avi@redhat.com>