utils: Deprecate hex-with-suffix sizes

Supporting '0x20M' looks odd, particularly since we have a 'B' suffix
that is ambiguous for bytes, as well as a less-frequently-used 'E'
suffix for extremely large exibytes.  In practice, people using hex
inputs are specifying values in bytes (and would have written
0x2000000, or possibly relied on default_suffix in the case of
qemu_strtosz_MiB), and the use of scaling suffixes makes the most
sense for inputs in decimal (where the user would write 32M).  But
rather than outright dropping support for hex-with-suffix, let's
follow our deprecation policy.  Sadly, since qemu_strtosz() does not
have an Err** parameter, and plumbing that in would be a much larger
task, we instead go with just directly emitting the deprecation
warning to stderr.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210211204438.1184395-4-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

docs/system/deprecated.rst

@@ -166,6 +166,14 @@ Using ``-M kernel-irqchip=off`` with x86 machine types that include a local
 APIC is deprecated.  The ``split`` setting is supported, as is using
 ``-M kernel-irqchip=off`` with the ISA PC machine type.
 
+hexadecimal sizes with scaling multipliers (since 6.0)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+Input parameters that take a size value should only use a size suffix
+(such as 'k' or 'M') when the base is written in decimal, and not when
+the value is hexadecimal.  That is, '0x20M' is deprecated, and should
+be written either as '32M' or as '0x2000000'.
+
 QEMU Machine Protocol (QMP) commands
 ------------------------------------