xl: introduce a firmware option

xl: introduce a firmware option

The new firmware option aims to provide a coherent way to set the
firmware for the different kind of guests Xen supports.

For PV guests the available firmwares are pvgrub{32|64}, and for HVM
the following are supported: bios, uefi, seabios, rombios and ovmf.
Note that uefi maps to ovmf, and bios maps to the default firmware for
each device model.

The xl.cfg man page is updated to document the new feature.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>