kvmclock: Don't crash QEMU if KVM is disabled

Most machines don't allow sysbus devices like "kvmclock" to be
created from the command-line, but some of them do (the ones with
has_dynamic_sysbus=true). In those cases, it's possible to
manually create a kvmclock device without KVM being enabled,
making QEMU crash:

  $ qemu-system-x86_64 -machine q35,accel=tcg -device kvmclock
  Segmentation fault (core dumped)

This changes kvmclock's realize method to return an error if KVM
is disabled, to ensure it won't crash QEMU.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170309185046.17555-1-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
