oeqa: Fix for QEMU_USE_KVM

oeqa: Fix for QEMU_USE_KVM

Fixed:
MACHINE = "qemux86"
QEMU_USE_KVM = "qemux86"
IMAGE_CLASSES += "testimage"

$ oe-selftest -r runqemu.RunqemuTests.test_boot_rootfs

[snip]
  File "/buildarea1/lyang1/poky/meta/lib/oe/types.py", line 122, in boolean
    raise ValueError("Invalid boolean value '%s'" % value)
ValueError: Invalid boolean value 'qemux86'

Now QEMU_USE_KVM can only be boolean, can not contain MACHINE any more, kvm
will be enabled if target_arch == build_arch or both of them are x86 archs.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>