bhyve: [snapshot] Do not flush readonly device at blockif_pause()

[gusev.vitaliy_gmail.com]

Summary

Part of Capsicum integration for snapshots.

Eliminates error:

blockif_pause: [WARN] failed to flush backing file.

Sponsored by: vStack

Test Plan

Compile, run VM, suspend/resume.

[markj]

usr.sbin/bhyve/block_if.c

1012

BTW, the BOP_FLUSH handler does not check whether the device is read-only before calling blockif_flush_bc(). Perhaps it should be a no-op in that situation?

[gusev.vitaliy_gmail.com]

usr.sbin/bhyve/block_if.c

1012

BOP_FLUSH() is called when emulated device directly calls blockif_flush(). Look at ahci_handle_flush() and nvme_opc_flush()) or pci_vtblk_proc().
In all cases it is just handle of guest's requests. So guest is aware about readonly device and shouldn't call flush commands.