qemu: blockjob: Add string convertors for blockjob type and state enums

qemu: blockjob: Add string convertors for blockjob type and state enums

Later on we'll format these values into the status XML so the from/to
string functions will come handy. The implementation also notes that
these will be used in the status XML to avoid somebody changing the
values.

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