KVM: x86: Ignore -EBUSY when checking nested events from vcpu_block()

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Ignore -EBUSY when checking nested events from vcpu_block()

Ignore -EBUSY when checking nested events after exiting a blocking state

while L2 is active, as exiting to userspace will generate a spurious

userspace exit, usually with KVM_EXIT_UNKNOWN, and likely lead to the VM's

demise. Continuing with the wakeup isn't perfect either, as *something*

has gone sideways if a vCPU is awakened in L2 with an injected event (or

worse, a nested run pending), but continuing on gives the VM a decent

chance of surviving without any major side effects.

As explained in the Fixes commits, it _should_ be impossible for a vCPU to

be put into a blocking state with an already-injected event (exception,

IRQ, or NMI). Unfortunately, userspace can stuff MP_STATE and/or injected

events, and thus put the vCPU into what should be an impossible state.

Don't bother trying to preserve the WARN, e.g. with an anti-syzkaller

Kconfig, as WARNs can (hopefully) be added in paths where _KVM_ would be

violating x86 architecture, e.g. by WARNing if KVM attempts to inject an

exception or interrupt while the vCPU isn't running.