qemu: Extract parsing of qemu namespace env vars into separate function

qemu: Extract parsing of qemu namespace env vars into separate function

Simplify the main function by splitting out how we parse the extra
passthrough environment variables.

Note that the validation function checks that the first letter must be a
character or underscore which makes the check whether the name is
redundant.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>