KVM: selftests: Complete x86_64/sync_regs_test ucall

The guest in sync_regs_test does raw ucalls by directly accessing the
ucall IO port. It makes these ucalls without setting %rdi to a `struct
ucall`, which is what a ucall uses to pass messages.  The issue is that
if the host did a get_ucall (the receiver side), it would try to access
the `struct ucall` at %rdi=0 which would lead to an error ("No mapping
for vm virtual address, gva: 0x0").

This issue is currently benign as there is no get_ucall in
sync_regs_test; however, that will change in the next commit as it
changes the unhandled exception reporting mechanism to use ucalls.  In
that case, every vcpu_run is followed by a get_ucall to check if the
guest is trying to report an unhandled exception.

Fix this in advance by setting %rdi to a UCALL_NONE struct ucall for the
sync_regs_test guest.

Tested with gcc-[8,9,10], and clang-[9,11].

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210611011020.3420067-3-ricarkol@google.com

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