KVM: selftests: Add string formatting options to ucall

Add more flexibility to guest debugging and testing by adding
GUEST_PRINTF() and GUEST_ASSERT_FMT() to the ucall framework.

Add a sized buffer to the ucall structure to hold the formatted string,
i.e. to allow the guest to easily resolve the string, and thus avoid the
ugly pattern of the host side having to make assumptions about the desired
format, as well as having to pass around a large number of parameters.

The buffer size was chosen to accommodate most use cases, and based on
similar usage.  E.g. printf() uses the same size buffer in
arch/x86/boot/printf.c.  And 1KiB ought to be enough for anybody.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
[sean: massage changelog, wrap macro param in ()]
Link: https://lore.kernel.org/r/20230729003643.1053367-8-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>