qemu_img: add cvtnum_full to print error reports

qemu_img: add cvtnum_full to print error reports

All calls to cvtnum check the return value and print the same error
message more or less. And so error reporting moved to cvtnum_full to
reduce code duplication and provide a single error
message. Additionally, cvtnum now wraps cvtnum_full with the existing
default range of 0 to MAX_INT64.

Acked-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: Eyal Moscovici <eyal.moscovici@oracle.com>
Message-Id: <20200513133629.18508-2-eyal.moscovici@oracle.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: fix printf formatting, avoid trailing space, change error wording,
reformat commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>