vmm: split errors in user and internal

Vmm errors can be either internal Firecracker errors (e.g. resource
exhaustion) or user errors (invalid paths for resources). Created
UserError and InternalError enums and Error has these two as variants.

Deleted Kernel, LogMetrics and Terminal errors as they were not
used.

Renamed VmSetup to VmConfigure as we have a similar error for Vcpus as
well.

Signed-off-by: Andreea Florescu <[email protected]>

Author: andreeaflorescu