x86/MSI: cleanup to prepare for multi-vector MSI

x86/MSI: cleanup to prepare for multi-vector MSI

The major aspect being the removal of the overload of the MSI entry's
mask_base field for MSI purposes - a proper union is being installed
instead, tracking both the config space position needed and the number
of vectors used (which is going to be 1 until the actual multi-vector
MSI patches arrive).

It also corrects misleading information from debug key 'M': When
msi_get_mask_bit() returns a negative value, there's no mask bit, and
hence output shouldn't give the impression there is.

Signed-off-by: Jan Beulich <jbeulich@suse.com>