doc: fix VM power manager guide as PDF

doc: fix VM power manager guide as PDF

When generating PDF with on an old system,
there are failures because of long tables:

vm_power_management.rst:420: ERROR: Malformed table.
Column span alignment problem in table line 5.
vm_power_management.rst:545: ERROR: Malformed table.
Column span alignment problem in table line 5.
vm_power_management.rst:754: ERROR: Malformed table.
Column span alignment problem in table line 5.

The tables (having only two columns) are replaced
with the more appropriate definition lists.

Fixes: 30d3aa861db5 ("doc: rework VM power manager user guide")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: David Marchand <david.marchand@redhat.com>