KVM: selftests: Add guest_snprintf() to KVM selftests

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>