kvm: selftests: tidy up kvm_util

Tidy up kvm-util code: code/comment formatting, remove unused code,
and move x86 specific code out. We also move vcpu_dump() out of
common code, because not all arches (AArch64) have KVM_GET_REGS.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>