KVM: selftests: Add guest udelay() utility for x86

Add udelay() for x86 tests to allow busy waiting in the guest for a
specific duration, and to match ARM and RISC-V's udelay() in the hopes
of eventually making udelay() available on all architectures.

Get the guest's TSC frequency using KVM_GET_TSC_KHZ and expose it to all
VMs via a new global, guest_tsc_khz.  Assert that KVM_GET_TSC_KHZ returns
a valid frequency, instead of simply skipping tests, which would require
detecting which tests actually need/want udelay().  KVM hasn't returned an
error for KVM_GET_TSC_KHZ since commit cc578287e322 ("KVM: Infrastructure
for software and hardware based TSC rate scaling"), which predates KVM
selftests by 6+ years (KVM_GET_TSC_KHZ itself predates KVM selftest by 7+
years).

Note, if the GUEST_ASSERT() in udelay() somehow fires and the test doesn't
check for guest asserts, then the test will fail with a very cryptic
message.  But fixing that, e.g. by automatically handling guest asserts,
is a much larger task, and practically speaking the odds of a test afoul
of this wart are infinitesimally small.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/r/5aa86285d1c1d7fe1960e3fe490f4b22273977e6.1718214999.git.reinette.chatre@intel.com
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>

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