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

[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>