[XEN] Extend dom0_mem syntax to support min/max clamps.

dom0_mem=[min:<min_amt>,][max:<max_amt>,][<amt>]

<min_amt>: The minimum amount of memory which should be allocated for dom0.
<max_amt>: The maximum amount of memory which should be allocated for dom0.
<amt>:     The precise amount of memory to allocate for dom0.

Notes:
 1. <amt> is clamped from below by <min_amt> and from above by available
    memory and <max_amt>
 2. <min_amt> is clamped from above by available memory and <max_amt>
 3. <min_amt> is ignored if it is greater than <max_amt>
 4. If <amt> is not specified, it is calculated as follows:
    "All of memory is allocated to domain 0, minus 1/16th which is reserved
     for uses such as DMA buffers (the reservation is clamped to 128MB)."

Each value can be specified as positive or negative:
 If +ve: The specified amount is an absolute value.
 If -ve: The specified amount is subtracted from total available memory.

Signed-off-by: Keir Fraser <keir@xensource.com>

docs/src/user.tex

@@ -3202,12 +3202,21 @@ editing \path{grub.conf}.
   ignored. This parameter may be specified with a B, K, M or G suffix,
   representing bytes, kilobytes, megabytes and gigabytes respectively.
   The default unit, if no suffix is specified, is kilobytes.
-\item [ dom0\_mem=xxx ] Set the amount of memory to be allocated to
-  domain0. In Xen 3.x the parameter may be specified with a B, K, M or
+\item [ dom0\_mem=$<$specifier list$>$ ] Set the amount of memory to
+  be allocated to domain 0. This is a comma-separated list containing
+  the following optional components:
+  \begin{description}
+  \item[ min:$<$min\_amt$>$ ] Minimum amount to allocate to domain 0
+  \item[ max:$<$min\_amt$>$ ] Maximum amount to allocate to domain 0
+  \item[ $<$amt$>$ ] Precise amount to allocate to domain 0
+  \end{description}
+  Each numeric parameter may be specified with a B, K, M or
   G suffix, representing bytes, kilobytes, megabytes and gigabytes
   respectively; if no suffix is specified, the parameter defaults to
-  kilobytes. In previous versions of Xen, suffixes were not supported
-  and the value is always interpreted as kilobytes.
+  kilobytes. Negative values are subtracted from total available
+  memory. If $<$amt$>$ is not specified, it defaults to all available
+  memory less a small amount (clamped to 128MB) for uses such as DMA
+  buffers.
 \item [ dom0\_vcpus\_pin ] Pins domain 0 VCPUs on their respective
   physical CPUS (default=false).
 \item [ tbuf\_size=xxx ] Set the size of the per-cpu trace buffers, in