KVM: selftests: Treat %llx like %lx when formatting guest printf

Treat %ll* formats the same as %l* formats when processing printfs from
the guest so that using e.g. %llx instead of %lx generates the expected
output. Ideally, unexpected formats would generate compile-time warnings
or errors, but it's not at all obvious how to actually accomplish that.

Alternatively, guest_vsnprintf() could assert on an unexpected format,
but since the vast majority of printfs are for failed guest asserts,
getting *something* printed is better than nothing.

E.g. before

==== Test Assertion Failure ====
x86_64/private_mem_conversions_test.c:265: mem[i] == 0
pid=4286 tid=4290 errno=4 - Interrupted system call
1 0x0000000000401c74: __test_mem_conversions at private_mem_conversions_test.c:336
2 0x00007f3aae6076da: ?? ??:0
3 0x00007f3aae32161e: ?? ??:0
Expected 0x0 at offset 0 (gpa 0x%lx), got 0x0

and after

==== Test Assertion Failure ====
x86_64/private_mem_conversions_test.c:265: mem[i] == 0
pid=5664 tid=5668 errno=4 - Interrupted system call
1 0x0000000000401c74: __test_mem_conversions at private_mem_conversions_test.c:336
2 0x00007fbe180076da: ?? ??:0
3 0x00007fbe17d2161e: ?? ??:0
Expected 0x0 at offset 0 (gpa 0x100000000), got 0xcc

Fixes: e5119382499c ("KVM: selftests: Add guest_snprintf() to KVM selftests")
Cc: Aaron Lewis <aaronlewis@google.com>
Link: https://lore.kernel.org/r/20230921171641.3641776-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

KVM: selftests: Add guest_snprintf() to KVM selftests

Add a local version of guest_snprintf() for use in the guest.

Having a local copy allows the guest access to string formatting
options without dependencies on LIBC. LIBC is problematic because
it heavily relies on both AVX-512 instructions and a TLS, neither of
which are guaranteed to be set up in the guest.

The file guest_sprintf.c was lifted from arch/x86/boot/printf.c and
adapted to work in the guest, including the addition of buffer length.
I.e. s/sprintf/snprintf/

The functions where prefixed with "guest_" to allow guests to
explicitly call them.

A string formatted by this function is expected to succeed or die. If
something goes wrong during the formatting process a GUEST_ASSERT()
will be thrown.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Link: https://lore.kernel.org/all/mtdi6smhur5rqffvpu7qux7mptonw223y2653x2nwzvgm72nlo@zyc4w3kwl3rg
[sean: add a link to the discussion of other options]
Link: https://lore.kernel.org/r/20230729003643.1053367-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>