core: Changed VdcBllErrors.InvalidPhysDev

Changed InvalidPhysDev to InaccessiblePhysDev
InvalidPhysDev gave a cryptic error message - Invalid physical device.
The message was amended into "Multipath cannot access physical device".
The error key was changed according to the new message content.
The error code remains the same. The corresponding exception was also
changed in vdsm - I648ee519873c51573e6e6306b79380f54bb25d2e , though
there's no dependency between the two patches (communicate through code,
which remains the same).

Change-Id: I50d91d159766ba9071f532fa56b6b90bd9d5d766
Bug-Url: https://bugzilla.redhat.com/852003
Signed-off-by: Vered Volansky <vvolansk@redhat.com>