KVM: x86: Move FPU allocation to common x86 code

The allocation of FPU structs is identical across VMX and SVM, move it
to common x86 code.  Somewhat arbitrarily place the allocation so that
it resides directly above the associated initialization via fx_init(),
e.g. instead of retaining its position with respect to the overall vcpu
creation flow.  Although the names names kvm_arch_vcpu_create() and
kvm_arch_vcpu_init() might suggest otherwise, x86 does not have a clean
split between 'create' and 'init'.  Allocating the struct immediately
prior to the first use arguably improves readability *now*, and will
yield even bigger improvements when kvm_arch_vcpu_init() is removed in
a future patch.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>