tests/vm: Only use -cpu 'host' if KVM is available

tests/vm: Only use -cpu 'host' if KVM is available

If KVM is not available, then use the 'max' cpu.

This fixes:

  ERROR:root:Log:
  ERROR:root:qemu-system-x86_64: CPU model 'host' requires KVM
  Failed to prepare guest environment
  error: [Errno 104] Connection reset by peer
  source/qemu/tests/vm/Makefile.include:25: recipe for target 'tests/vm/ubuntu.i386.img' failed
  make: *** [tests/vm/ubuntu.i386.img] Error 2

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180628153535.1411-4-f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>