igb: fix VF start with PF stopped

igb: fix VF start with PF stopped

When igb runs as a PF, mbox interrupt is prerequisite to make VF
start normally.
And PF sometimes won't 'dev_start', so the mbox interrupt register
during 'dev_init' is required.
The patch rolls back the interrupt register for mbox,lsc to the 'dev_init'.
As UIO doesn't support multiple vector, mbox has to occupy the only one.
It adds condition check on 'dev_start', rxq interrupt is not allowed
when PF running in IOV mode via UIO.

Signed-off-by: Cunming Liang <cunming.liang@intel.com>