When adding hostmem backend at runtime, QEMU might exit with error:
  "os_mem_prealloc: Insufficient free host memory pages available to allocate guest RAM"

It happens due to os_mem_prealloc() not handling errors gracefully.

Fix it by passing errp argument so that os_mem_prealloc() could
report error to callers and undo performed allocation when
os_mem_prealloc() fails.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1469008443-72059-1-git-send-email-imammedo@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>