x86: use a non-negative number in shift

Shifting a negative number is undefined. Clang complains about it:

x86/svm.c:1131:38: error: shifting a negative signed value is undefined [-Werror,-Wshift-negative-value]
    test->vmcb->control.tsc_offset = TSC_OFFSET_VALUE;

Using "~0ull" results in identical asm code:

	before: movabsq $-281474976710656, %rsi
	after:  movabsq $-281474976710656, %rsi

Signed-off-by: Bill Wendling <morbo@google.com>
[thuth: Rebased to master - code is in svm_tests.c instead of svm.c now]
Message-Id: <20200514192626.9950-9-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>