target-i386: Reorganize TSC rate setting code

target-i386: Reorganize TSC rate setting code

Following changes are made to the TSC rate setting code in
kvm_arch_init_vcpu():
 * The code is moved to a new function kvm_arch_set_tsc_khz().
 * If kvm_arch_set_tsc_khz() fails, i.e. following two conditions are
   both satisfied:
   * KVM does not support the TSC scaling or it fails to set vcpu's
     TSC rate by KVM_SET_TSC_KHZ,
   * the TSC rate to be set is different than the value currently used
     by KVM, then kvm_arch_init_vcpu() will fail. Prevously,
   * the lack of TSC scaling never failed kvm_arch_init_vcpu(),
   * the failure of KVM_SET_TSC_KHZ failed kvm_arch_init_vcpu()
     unconditionally, even though the TSC rate to be set is identical
     to the value currently used by KVM.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>