msi: remove the check that interrupt sources have been added

When running as a specific type of Xen guest the hypervisor won't
provide any emulated IO-APICs or legacy PICs at all, thus hitting the
following assert in the MSI code:

panic: Assertion num_io_irqs > 0 failed at /usr/src/sys/x86/x86/msi.c:334
cpuid = 0
time = 1
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffffff826ffa70
vpanic() at vpanic+0x1a3/frame 0xffffffff826ffad0
panic() at panic+0x43/frame 0xffffffff826ffb30
msi_init() at msi_init+0xed/frame 0xffffffff826ffb40
apic_setup_io() at apic_setup_io+0x72/frame 0xffffffff826ffb50
mi_startup() at mi_startup+0x118/frame 0xffffffff826ffb70
start_kernel() at start_kernel+0x10

Fix this by removing the assert in the MSI code, since it's possible
to get to the MSI initialization without having registered any other
interrupt sources.

Reviewed by: jhb
Approved by: re (gjb)
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D17001