virtio-msg: remove emph or textbf for MUST/MAY/SHOULD

To comply with the rest of the OASIS Virtio specification removing all
emph or textbf around MUST/MAY/SHOULD.
Do some minor rewording to prevent using normative wording when not
required.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>

Author: bertrand-marquis