arch-x86: Improve KVM set XCR

[abmerop]

This adds two failsafes which may cause a panic on some machines. First, check the host machine has the KVM XCR capability before calling getXCRs or setXCRs. Second, ensure the x87 bit, which must always be one, will always return at least one by modifying the return value in readMiscReg.

Change-Id: I5e778acc926a47443ef6cef29fabd84eb69bb9ba