webadmin: Changed units of Network QoS in UI

Network QoS 'Burst' property is commonly counted in Megabytes and not
Megabits. vdsm API also receives this value in Megabytes.

UI was changed (label) to support this convention and tooltip added to
point the difference between 'Burst' and 'Average' and 'Peak' fields.

Change-Id: I1dd2367b136d547f7f93fdc8a1b76561920ce59a
Bug-Url: https://bugzilla.redhat.com/999799
Signed-off-by: Ofri Masad <omasad@redhat.com>