conf: validate: Move qemu-specific LUN disk validation to global validation

conf: validate: Move qemu-specific LUN disk validation to global validation

LUN disks are supported only by VMX and QEMU drivers and the VMX
implementation is a subset of qemu's implementation, thus we can move
the qemu-specific validator to the global validation code providing that
we allow the format to be 'none' (qemu driver always sets 'raw' if it's
not set) and allow disk type 'volume' as a source (qemu always
translates the source, and VMX doesn't implement 'volume' at all).

Moving the code to the global validation allows us to stop calling it
from the qemu specific validation and also deduplicates the checks.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>