throttle: Make burst_length 64bit and add range checks

LeakyBucket.burst_length is defined as an unsigned integer but the
code never checks for overflows and it only makes sure that the value
is not 0.

In practice this means that the user can set something like
throttling.iops-total-max-length=4294967300 despite being larger than
UINT_MAX and the final value after casting to unsigned int will be 4.

This patch changes the data type to uint64_t. This does not increase
the storage size of LeakyBucket, and allows us to assign the value
directly from qemu_opt_get_number() or BlockIOThrottle and then do the
checks directly in throttle_is_valid().

The value of burst_length does not have a specific upper limit,
but since the bucket size is defined by max * burst_length we have
to prevent overflows. Instead of going for UINT64_MAX or something
similar this patch reuses THROTTLE_VALUE_MAX, which allows I/O bursts
of 1 GiB/s for 10 days in a row.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 1b2e3049803f71cafb2e1fa1be4fb47147a0d398.1503580370.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
